fix some issues and add test Blazor frontend

ColorControl.UI/State/AppState.cs

@@ -0,0 +1,16 @@
+namespace ColorControl.UI;
+
+public class AppState
+{
+    public string SelectedTheme { get; private set; } = "light";
+
+    public event Action? OnChange;
+
+    public void SetTheme(string theme)
+    {
+        SelectedTheme = theme;
+        NotifyStateChanged();
+    }
+
+    private void NotifyStateChanged() => OnChange?.Invoke();
+}