refactor(ui): simplify dark theme to monochrome grayscale

Replace vibrant glassmorphic design with clean monochrome palette where
dark theme is the grayscale inversion of light theme.

Changes:
- Remove all vibrant accent colors (cyan, emerald, rose, amber)
- Dark theme now uses black/white/greys inverted from light theme
- Light theme colors remain unchanged
- Remove opacity-based glass layers, use solid greys
- Update JToggle to use Primary/Secondary instead of Accent
- Simplify comments to reflect new monochrome approach

Color mapping (dark theme inverted from light):
- BgBase: #0F1419 (near-black) ← #FFFFFF (white)
- BgSurface: #374151 (gray-700) ← #E5E7EB (gray-200)
- TextPrimary: #FFFFFF (white) ← #111827 (gray-900)
- Primary: #D1D5DB (gray-300) ← #374151 (gray-700)
- Borders: #4B5563 (gray-600) ← #D1D5DB (gray-300)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>
Signed-off-by: JasonXuDeveloper - 傑 <jason@xgamedev.net>

Author: JasonXuDeveloper

UnityProject/Packages/com.jasonxudeveloper.jengine.ui/Editor/Components/Form/JToggle.cs

@@ -45,15 +45,15 @@ public class JToggle : VisualElement
 
         // Theme-aware colors
         private static Color TrackOffColor => Tokens.IsDarkTheme
-            ? new Color(0.24f, 0.24f, 0.30f, 1f)  // Dark grey in dark mode
-            : Tokens.Colors.Secondary;              // Medium grey (darker) in light mode
+            ? Tokens.Colors.Secondary               // Medium-dark grey (gray-600) in dark mode
+            : Tokens.Colors.Secondary;              // Medium grey (gray-300) in light mode
 
         private static Color TrackOnColor => Tokens.IsDarkTheme
-            ? Tokens.Colors.Accent                  // Cyan in dark mode
-            : Tokens.Colors.Primary;                // Dark grey in light mode
+            ? Tokens.Colors.Primary                 // Light grey (gray-300) in dark mode - inverted
+            : Tokens.Colors.Primary;                // Dark grey (gray-700) in light mode
 
         private static Color ThumbColor => Tokens.IsDarkTheme
-            ? new Color(0.88f, 0.88f, 0.88f, 1f)  // Light grey in dark mode
+            ? Tokens.Colors.TextPrimary             // White in dark mode
             : Color.white;                          // Pure white in light mode
 
         /// <summary>

UnityProject/Packages/com.jasonxudeveloper.jengine.ui/Editor/Components/Layout/JCard.cs

@@ -34,11 +34,11 @@ namespace JEngine.UI.Editor.Components.Layout
     public class JCard : JComponent
     {
         /// <summary>
-        /// Creates a new glassmorphic card container with light reflection borders.
+        /// Creates a new card container with subtle borders.
         /// </summary>
         public JCard() : base("j-card")
         {
-            // Apply glassmorphic card styling using JTheme helper
+            // Apply card styling using JTheme helper
             JTheme.ApplyGlassCard(this);
 
             // Set padding

UnityProject/Packages/com.jasonxudeveloper.jengine.ui/Editor/Theming/JTheme.cs

@@ -106,7 +106,7 @@ public static void ApplySubtle(VisualElement element)
         }
 
         /// <summary>
-        /// Applies glassmorphic border with light reflection effect.
+        /// Applies borders with theme-appropriate colors.
         /// </summary>
         /// <param name="element">The element to apply borders to.</param>
         /// <param name="subtle">Use subtle borders if true.</param>
@@ -121,7 +121,7 @@ public static void ApplyBorder(VisualElement element, bool subtle = false)
             }
             else
             {
-                // Light reflection: top/left lighter, bottom/right darker
+                // Uniform borders using BorderLight/BorderDark (same in monochrome)
                 element.style.borderTopColor = Tokens.Colors.BorderLight;
                 element.style.borderLeftColor = Tokens.Colors.BorderLight;
                 element.style.borderBottomColor = Tokens.Colors.BorderDark;
@@ -158,9 +158,9 @@ public static void ApplyTransition(VisualElement element)
         }
 
         /// <summary>
-        /// Applies glassmorphic card style (surface + borders + transitions).
+        /// Applies card style (surface + borders + transitions).
         /// </summary>
-        /// <param name="element">The element to style as a glass card.</param>
+        /// <param name="element">The element to style as a card.</param>
         public static void ApplyGlassCard(VisualElement element)
         {
             ApplySurface(element);