diff --git a/frontend/src/components/Layout/Sidebar/index.tsx b/frontend/src/components/Layout/Sidebar/index.tsx
index 49a0817..96b3505 100644
--- a/frontend/src/components/Layout/Sidebar/index.tsx
+++ b/frontend/src/components/Layout/Sidebar/index.tsx
@@ -1,6 +1,7 @@
 import { useState } from "react";
 import { useTranslation } from "react-i18next";
 import { Link as RouterLink, useLocation, useNavigate } from "react-router-dom";
+import type { Theme } from "@mui/material/styles";
 import Box from "@mui/material/Box";
 import Link from "@mui/material/Link";
 import Stack from "@mui/material/Stack";
@@ -32,6 +33,24 @@ const SIDEBAR_WIDTH = 240;
 const SIDEBAR_COLLAPSED_WIDTH = 72;
 const SIDEBAR_COLLAPSED_STORAGE_KEY = "thingport_sidebar_collapsed";
 
+/** Shared color/background logic for every nav row (both the expanded ListItemButton and the
+ *  collapsed icon-only variant below) -- selected rows get the theme's nav-selected background
+ *  (a flat tint in light mode, a left-to-right gradient in dark) and text/icon color, unselected
+ *  ones get the theme's dedicated (narrower-than-text.secondary) inactive nav color. */
+function navRowSx(selected: boolean) {
+  const color = (theme: Theme) => (selected ? theme.thingport.selectedNavText : theme.thingport.navInactiveText);
+  return {
+    color,
+    "& .MuiListItemIcon-root": { color },
+    ...(selected && {
+      background: (theme: Theme) => theme.thingport.selectedNavBackground,
+      "&.Mui-selected, &.Mui-selected:hover": {
+        background: (theme: Theme) => theme.thingport.selectedNavBackground,
+      },
+    }),
+  };
+}
+
 /** Icon-only rail row used for every nav item once the sidebar is collapsed -- a tooltip stands
  *  in for the label. */
 function CollapsedNavIcon({ icon, label, selected, onClick }: {
@@ -45,9 +64,9 @@ function CollapsedNavIcon({ icon, label, selected, onClick }: {
       <ListItemButton
         selected={selected}
         onClick={onClick}
-        sx={{ borderRadius: 1, mb: 0.5, justifyContent: "center", px: 0 }}
+        sx={{ borderRadius: 1, mb: 0.5, justifyContent: "center", px: 0, ...navRowSx(selected) }}
       >
-        <ListItemIcon sx={{ minWidth: 0, color: selected ? "primary.main" : "text.secondary" }}>
+        <ListItemIcon sx={{ minWidth: 0 }}>
           {icon}
         </ListItemIcon>
       </ListItemButton>
@@ -116,8 +135,6 @@ export default function Sidebar({ isAdmin, onSelectCategory }: Props) {
         top: 0,
         display: "flex",
         flexDirection: "column",
-        borderRight: "1px solid",
-        borderColor: "divider",
         bgcolor: "background.paper",
         overflow: "hidden",
         transition: (theme) => theme.transitions.create("width", { duration: theme.transitions.duration.shortest }),
@@ -156,7 +173,7 @@ export default function Sidebar({ isAdmin, onSelectCategory }: Props) {
               onClick={() => navigate("/")}
             />
           ) : (
-            <ListItemButton selected={onDashboard} onClick={() => navigate("/")} sx={{ borderRadius: 1, mb: 0.5 }}>
+            <ListItemButton selected={onDashboard} onClick={() => navigate("/")} sx={{ borderRadius: 1, mb: 0.5, ...navRowSx(onDashboard) }}>
               <ListItemIcon sx={{ minWidth: 30 }}>
                 <SpaceDashboardIcon fontSize="small" />
               </ListItemIcon>
@@ -175,7 +192,7 @@ export default function Sidebar({ isAdmin, onSelectCategory }: Props) {
             <ListItemButton
               selected={onModels}
               onClick={goToModelsRoot}
-              sx={{ borderRadius: 1, mb: 0.5 }}
+              sx={{ borderRadius: 1, mb: 0.5, ...navRowSx(onModels) }}
             >
               <ListItemIcon sx={{ minWidth: 30 }}>
                 <ViewInArIcon fontSize="small" />
@@ -195,7 +212,7 @@ export default function Sidebar({ isAdmin, onSelectCategory }: Props) {
             <ListItemButton
               selected={onCollections}
               onClick={() => navigate("/models/collections")}
-              sx={{ borderRadius: 1, mb: 0.5 }}
+              sx={{ borderRadius: 1, mb: 0.5, ...navRowSx(onCollections) }}
             >
               <ListItemIcon sx={{ minWidth: 30 }}>
                 <CollectionsIcon fontSize="small" />
@@ -215,7 +232,7 @@ export default function Sidebar({ isAdmin, onSelectCategory }: Props) {
             <ListItemButton
               selected={onDownload}
               onClick={() => navigate("/downloads")}
-              sx={{ borderRadius: 1, mb: 0.5 }}
+              sx={{ borderRadius: 1, mb: 0.5, ...navRowSx(onDownload) }}
             >
               <ListItemIcon sx={{ minWidth: 30 }}>
                 <DownloadIcon fontSize="small" />
@@ -265,7 +282,7 @@ export default function Sidebar({ isAdmin, onSelectCategory }: Props) {
                   <ListItemButton
                     selected={false}
                     onClick={() => setAdminExpanded(v => !v)}
-                    sx={{ borderRadius: 1, mb: 0.5 }}
+                    sx={{ borderRadius: 1, mb: 0.5, ...navRowSx(onAdmin) }}
                   >
                     <ListItemIcon sx={{ minWidth: 30 }}>
                       <AdminPanelSettingsIcon fontSize="small" />
@@ -278,7 +295,7 @@ export default function Sidebar({ isAdmin, onSelectCategory }: Props) {
                       <ListItemButton
                         selected={onAdminSettings}
                         onClick={() => navigate("/admin-settings")}
-                        sx={{ borderRadius: 1, mb: 0.5, pl: 4 }}
+                        sx={{ borderRadius: 1, mb: 0.5, pl: 4, ...navRowSx(onAdminSettings) }}
                       >
                         <ListItemIcon sx={{ minWidth: 30 }}>
                           <SettingsIcon fontSize="small" />
@@ -288,7 +305,7 @@ export default function Sidebar({ isAdmin, onSelectCategory }: Props) {
                       <ListItemButton
                         selected={onAdminUsers}
                         onClick={() => navigate("/admin-users")}
-                        sx={{ borderRadius: 1, mb: 0.5, pl: 4 }}
+                        sx={{ borderRadius: 1, mb: 0.5, pl: 4, ...navRowSx(onAdminUsers) }}
                       >
                         <ListItemIcon sx={{ minWidth: 30 }}>
                           <PeopleIcon fontSize="small" />
@@ -298,7 +315,7 @@ export default function Sidebar({ isAdmin, onSelectCategory }: Props) {
                       <ListItemButton
                         selected={onAdminLogs}
                         onClick={() => navigate("/admin-logs")}
-                        sx={{ borderRadius: 1, mb: 0.5, pl: 4 }}
+                        sx={{ borderRadius: 1, mb: 0.5, pl: 4, ...navRowSx(onAdminLogs) }}
                       >
                         <ListItemIcon sx={{ minWidth: 30 }}>
                           <HistoryIcon fontSize="small" />
@@ -308,7 +325,7 @@ export default function Sidebar({ isAdmin, onSelectCategory }: Props) {
                       <ListItemButton
                         selected={onAdminTriggers}
                         onClick={() => navigate("/admin-triggers")}
-                        sx={{ borderRadius: 1, mb: 0.5, pl: 4 }}
+                        sx={{ borderRadius: 1, mb: 0.5, pl: 4, ...navRowSx(onAdminTriggers) }}
                       >
                         <ListItemIcon sx={{ minWidth: 30 }}>
                           <BoltIcon fontSize="small" />
@@ -318,7 +335,7 @@ export default function Sidebar({ isAdmin, onSelectCategory }: Props) {
                       <ListItemButton
                         selected={onAdminConnections}
                         onClick={() => navigate("/admin-connections")}
-                        sx={{ borderRadius: 1, mb: 0.5, pl: 4 }}
+                        sx={{ borderRadius: 1, mb: 0.5, pl: 4, ...navRowSx(onAdminConnections) }}
                       >
                         <ListItemIcon sx={{ minWidth: 30 }}>
                           <CableIcon fontSize="small" />
diff --git a/frontend/src/pages/CollectionsPage/CollectionCard.tsx b/frontend/src/pages/CollectionsPage/CollectionCard.tsx
index f5f2943..f73c21b 100644
--- a/frontend/src/pages/CollectionsPage/CollectionCard.tsx
+++ b/frontend/src/pages/CollectionsPage/CollectionCard.tsx
@@ -88,7 +88,7 @@ export default function CollectionCard({ collection, theme, previewMode, onUpdat
         overflow: "hidden",
         borderRadius: "12px",
         borderColor: "divider",
-        bgcolor: (muiTheme) => (muiTheme.palette.mode === "dark" ? muiTheme.palette.grey[800] : muiTheme.palette.grey[100]),
+        bgcolor: (muiTheme) => (muiTheme.palette.mode === "dark" ? muiTheme.thingport.pageBackground : muiTheme.palette.grey[100]),
         "&:hover .collection-card-actions": { opacity: 1 },
       }}
     >
@@ -96,7 +96,7 @@ export default function CollectionCard({ collection, theme, previewMode, onUpdat
         sx={{
           width: "100%",
           aspectRatio: "4 / 3",
-          bgcolor: (muiTheme) => (muiTheme.palette.mode === "dark" ? muiTheme.palette.grey[900] : muiTheme.palette.grey[200]),
+          bgcolor: (muiTheme) => (muiTheme.palette.mode === "dark" ? muiTheme.thingport.pageBackground : muiTheme.palette.grey[200]),
         }}
       >
         {coverItems.length === 0 && (
diff --git a/frontend/src/pages/ModelDetailPage/ModelSidePanel.tsx b/frontend/src/pages/ModelDetailPage/ModelSidePanel.tsx
index 86bea89..1a5fd56 100644
--- a/frontend/src/pages/ModelDetailPage/ModelSidePanel.tsx
+++ b/frontend/src/pages/ModelDetailPage/ModelSidePanel.tsx
@@ -60,7 +60,7 @@ export default function ModelSidePanel({ print, onSelectCategory, onUnauthorized
 
   return (
     <Paper
-      variant="outlined"
+      elevation={0}
       sx={{
         p: 2.5,
         borderRadius: "12px",
diff --git a/frontend/src/pages/ModelsPage/CategoriesPanel.tsx b/frontend/src/pages/ModelsPage/CategoriesPanel.tsx
index 2205192..6b58cf4 100644
--- a/frontend/src/pages/ModelsPage/CategoriesPanel.tsx
+++ b/frontend/src/pages/ModelsPage/CategoriesPanel.tsx
@@ -1,5 +1,6 @@
 import { useMemo, useState } from "react";
 import { useTranslation } from "react-i18next";
+import type { Theme } from "@mui/material/styles";
 import Paper from "@mui/material/Paper";
 import Stack from "@mui/material/Stack";
 import Typography from "@mui/material/Typography";
@@ -28,6 +29,31 @@ type Props = {
   onUpdateMeta: (id: string, meta: CategoryMetaInput) => Promise<void>;
 };
 
+// Every row (the pinned "All" row, a root category, or one of its children) is either the active
+// filter (the theme's nav-selected gradient + accent text) or not (transparent, the theme's
+// dedicated nav-inactive gray) -- a plain two-state system, rather than the extra "expanded but
+// not the active one" light-grey/dark-grey tiers this used to hardcode, which were literal
+// light-theme hex values (#ffffff, #5c5c5c, #212b36, #a3a3a3, and a translucent near-white row
+// background) that read as invisible or wrong against a dark background.
+function rowSx(active: boolean) {
+  return {
+    borderRadius: 1.5,
+    mb: 0.25,
+    background: (theme: Theme) => (active ? theme.thingport.selectedNavBackground : "transparent"),
+    "&:hover": { bgcolor: "action.hover" },
+  };
+}
+function rowTextSx(active: boolean, extra?: object) {
+  return {
+    noWrap: true,
+    variant: "body2" as const,
+    sx: {
+      color: (theme: Theme) => (active ? theme.thingport.selectedNavText : theme.thingport.navInactiveText),
+      ...extra,
+    },
+  };
+}
+
 /** The Models page's own category browser: a pinned "All" row, then a strictly two-level tree --
  *  top-level categories expand to reveal their subcategories AND select themselves, filtering the
  *  grid to every model under any of their subcategories; subcategories narrow the filter down to
@@ -71,7 +97,7 @@ export default function CategoriesPanel({ categories, loading, selectedId, onSel
   return (
     <>
       <Paper
-        variant="outlined"
+        elevation={0}
         sx={{
           width: 260,
           flexShrink: 0,
@@ -91,16 +117,11 @@ export default function CategoriesPanel({ categories, loading, selectedId, onSel
         </Stack>
 
         <List disablePadding>
-          <ListItemButton
-            onClick={() => onSelect(null)}
-            sx={{
-              borderRadius: 1.5,
-              mb: 0.25,
-              bgcolor: selectedId === null ? "action.selected" : "transparent",
-              "&:hover": { bgcolor: "background.default" },
-            }}
-          >
-            <ListItemText primary={t("models:categories.all")} primaryTypographyProps={{ variant: "body2", fontWeight: 600 }} />
+          <ListItemButton onClick={() => onSelect(null)} sx={rowSx(selectedId === null)}>
+            <ListItemText
+              primary={t("models:categories.all")}
+              primaryTypographyProps={rowTextSx(selectedId === null, { fontWeight: 600 })}
+            />
           </ListItemButton>
 
           {loading && (
@@ -115,23 +136,10 @@ export default function CategoriesPanel({ categories, loading, selectedId, onSel
             const isRootActive = isOpen && selectedId === root.id;
             return (
               <Stack key={root.id}>
-                <ListItemButton
-                  onClick={() => handleRootClick(root.id)}
-                  sx={{
-                    borderRadius: 1.5,
-                    mb: 0.25,
-                    bgcolor: isOpen ? (isRootActive ? "rgba(0, 174, 66, 0.08)" : "#ffffff") : "transparent",
-                    "&:hover": { bgcolor: isRootActive ? "rgba(0, 174, 66, 0.08)" : "background.default" },
-                  }}
-                >
+                <ListItemButton onClick={() => handleRootClick(root.id)} sx={rowSx(isRootActive)}>
                   <ListItemText
                     primary={displayName(root) || untitledLabel}
-                    primaryTypographyProps={{
-                      noWrap: true,
-                      variant: "body2",
-                      fontWeight: 600,
-                      sx: { color: isOpen ? (isRootActive ? "primary.main" : "#5c5c5c") : "#212b36" },
-                    }}
+                    primaryTypographyProps={rowTextSx(isRootActive, { fontWeight: 600 })}
                   />
                   <ChevronRightIcon
                     fontSize="small"
@@ -140,7 +148,7 @@ export default function CategoriesPanel({ categories, loading, selectedId, onSel
                       flexShrink: 0,
                       transform: isOpen ? "rotate(90deg)" : "none",
                       transition: "transform 0.15s",
-                      color: isOpen ? (isRootActive ? "primary.main" : "#a3a3a3") : "#a3a3a3",
+                      color: (theme) => (isRootActive ? theme.thingport.selectedNavText : theme.thingport.navInactiveText),
                     }}
                   />
                 </ListItemButton>
@@ -152,21 +160,11 @@ export default function CategoriesPanel({ categories, loading, selectedId, onSel
                         <ListItemButton
                           key={child.id}
                           onClick={() => onSelect(child.id)}
-                          sx={{
-                            pl: 4,
-                            borderRadius: 1.5,
-                            mb: 0.25,
-                            bgcolor: "rgba(242, 242, 242, 0.3)",
-                            "&:hover": { bgcolor: "background.default" },
-                          }}
+                          sx={{ pl: 4, ...rowSx(isChildSelected) }}
                         >
                           <ListItemText
                             primary={displayName(child) || untitledLabel}
-                            primaryTypographyProps={{
-                              noWrap: true,
-                              variant: "body2",
-                              sx: isChildSelected ? { color: "primary.main", fontWeight: 700 } : undefined,
-                            }}
+                            primaryTypographyProps={rowTextSx(isChildSelected, isChildSelected ? { fontWeight: 700 } : undefined)}
                           />
                         </ListItemButton>
                       );
diff --git a/frontend/src/pages/ModelsPage/ModelCard.tsx b/frontend/src/pages/ModelsPage/ModelCard.tsx
index 6708cf3..68c50cf 100644
--- a/frontend/src/pages/ModelsPage/ModelCard.tsx
+++ b/frontend/src/pages/ModelsPage/ModelCard.tsx
@@ -100,7 +100,7 @@ export default function ModelCard({
         overflow: "hidden",
         borderRadius: "12px",
         borderColor: "transparent",
-        bgcolor: (muiTheme) => (muiTheme.palette.mode === "dark" ? muiTheme.palette.grey[800] : muiTheme.palette.grey[100]),
+        bgcolor: (muiTheme) => (muiTheme.palette.mode === "dark" ? muiTheme.thingport.pageBackground : muiTheme.palette.grey[100]),
         transition: "background-color .15s ease, box-shadow .15s ease, transform .15s ease",
         "&:hover": {
           bgcolor: "background.paper",
@@ -115,7 +115,7 @@ export default function ModelCard({
         sx={{
           width: "100%",
           aspectRatio: "4 / 3",
-          bgcolor: (muiTheme) => (muiTheme.palette.mode === "dark" ? muiTheme.palette.grey[900] : muiTheme.palette.grey[200]),
+          bgcolor: (muiTheme) => (muiTheme.palette.mode === "dark" ? muiTheme.thingport.pageBackground : muiTheme.palette.grey[200]),
         }}
       >
         {renderPreviewContent(item, "card", theme, t, previewMode)}
diff --git a/frontend/src/theme.ts b/frontend/src/theme.ts
index ee9d18c..1ec0932 100644
--- a/frontend/src/theme.ts
+++ b/frontend/src/theme.ts
@@ -1,4 +1,4 @@
-import { alpha, createTheme, darken, lighten, type Theme, type ThemeOptions } from "@mui/material/styles";
+import { alpha, createTheme, darken, type Theme, type ThemeOptions } from "@mui/material/styles";
 import type { ResolvedTheme } from "./constants/settingsOptions";
 
 export type { ResolvedTheme };
@@ -26,6 +26,17 @@ declare module "@mui/material/styles" {
        *  (e.g. an attention/alert-style label) -- not one of MUI's error/warning palette
        *  roles, just a second brand-adjacent color for cases that call for it. */
       altText: string;
+      /** Unselected nav-row label/icon color for the Sidebar rail and the Models page's
+       *  Categories box -- narrower than text.secondary (which is used all over for ordinary
+       *  muted text) so recoloring nav rows can't leak into unrelated UI. */
+      navInactiveText: string;
+      /** Selected nav row background (Sidebar rail rows, Categories box rows) -- a `background`
+       *  (not `bgcolor`) value since dark mode's is a left-to-right gradient, not a flat color;
+       *  light mode's is still just a flat tint expressed the same way. */
+      selectedNavBackground: string;
+      /** Selected nav row label/icon color -- the brand accent in dark mode, unchanged
+       *  (inherited) text color in light mode. */
+      selectedNavText: string;
     };
   }
   interface ThemeOptions {
@@ -34,6 +45,9 @@ declare module "@mui/material/styles" {
       modelColor: string;
       modelEmissive: string;
       altText: string;
+      navInactiveText: string;
+      selectedNavBackground: string;
+      selectedNavText: string;
     };
   }
 }
@@ -56,6 +70,9 @@ type ThemeDef = {
   altText: string;
   modelColor: string;
   modelEmissive: string;
+  navInactiveText: string;
+  selectedNavBackground: string;
+  selectedNavText: string;
 };
 
 // Values ported 1:1 from the original CSS custom properties so the visual identity of each
@@ -79,25 +96,32 @@ const THEME_DEFS: Record<ResolvedTheme, ThemeDef> = {
     altText: "rgb(255, 114, 32)",
     modelColor: "#cbd5e1",
     modelEmissive: "#94a3b8",
+    navInactiveText: "#858585",
+    selectedNavBackground: alpha("#5be584", 0.2),
+    // Matches the pre-existing collapsed-rail icon treatment (selected -> primary.main).
+    selectedNavText: "#00b800",
   },
   dark: {
     mode: "dark",
-    pageBackground: "#0a0b0c",
-    panel: "rgba(15, 23, 42, 0.86)",
-    panelStrong: "#0f172a",
-    border: "#1f2937",
-    borderStrong: "#334155",
-    text: "#f8fafc",
-    textMuted: "#94a3b8",
-    textSubtle: "#64748b",
-    accent: "#34d399",
-    accentLight: lighten("#34d399", 0.25),
-    accentStrong: "#10b981",
-    accentSoft: "rgba(16, 185, 129, 0.2)",
+    pageBackground: "#181f39",
+    panel: "#1e2746",
+    panelStrong: "#1e2746",
+    border: "#333a54",
+    borderStrong: "#333a54",
+    text: "#828690",
+    textMuted: "#828690",
+    textSubtle: "#828690",
+    accent: "#00b800",
+    accentLight: "#5be584",
+    accentStrong: darken("#00b800", 0.15),
+    accentSoft: alpha("#00b800", 0.16),
     accentContrast: "#ffffff",
     altText: "rgb(255, 114, 32)",
     modelColor: "#e2e8f0",
     modelEmissive: "#475569",
+    navInactiveText: "#969ba0",
+    selectedNavBackground: "linear-gradient(to right, rgb(24, 31, 57) 0%, rgba(49, 206, 255, 0) 100%)",
+    selectedNavText: "#00b800",
   },
 };
 
@@ -106,7 +130,7 @@ export function buildTheme(id: ResolvedTheme): Theme {
   const options: ThemeOptions = {
     palette: {
       mode: d.mode,
-      background: { default: d.mode === "light" ? d.pageBackground : d.panelStrong, paper: d.panel },
+      background: { default: d.pageBackground, paper: d.panel },
       primary: { main: d.accent, light: d.accentLight, dark: d.accentStrong, contrastText: d.accentContrast },
       text: { primary: d.text, secondary: d.textMuted },
       divider: d.border,
@@ -115,7 +139,7 @@ export function buildTheme(id: ResolvedTheme): Theme {
         hover: alpha(d.accent, 0.08),
       },
     },
-    shape: { borderRadius: 10 },
+    shape: { borderRadius: d.mode === "dark" ? 0 : 10 },
     typography: {
       fontSize: 13,
       fontFamily: BODY_FONT_STACK,
@@ -130,12 +154,23 @@ export function buildTheme(id: ResolvedTheme): Theme {
       MuiButton: { styleOverrides: { root: { borderRadius: 8 } } },
       MuiChip: { styleOverrides: { root: { borderRadius: 6 } } },
       MuiTooltip: { styleOverrides: { tooltip: { backgroundColor: d.panelStrong, color: d.text } } },
+      // Dark mode is deliberately square everywhere -- corners, chips, avatars, the back-to-top
+      // FAB, all of it. `sx`-set radii (the vast majority of them: cards, panels, the info box,
+      // etc.) beat theme.shape.borderRadius and component styleOverrides alike, so nothing short
+      // of an !important global rule reaches all of them; MuiCssBaseline's styleOverrides is
+      // exactly the escape hatch for page-level CSS the theme needs to own like this.
+      ...(d.mode === "dark"
+        ? { MuiCssBaseline: { styleOverrides: "*, *::before, *::after { border-radius: 0 !important; }" } }
+        : {}),
     },
     thingport: {
       pageBackground: d.pageBackground,
       modelColor: d.modelColor,
       modelEmissive: d.modelEmissive,
       altText: d.altText,
+      navInactiveText: d.navInactiveText,
+      selectedNavBackground: d.selectedNavBackground,
+      selectedNavText: d.selectedNavText,
     },
   };
   return createTheme(options);