Fix dark theme detection using IThemeEngine API

Replace preference-based theme detection with the proper IThemeEngine
service API. The previous approach read the theme ID from preferences
which could be stale or unavailable. The new approach queries the
active theme directly via IThemeEngine.getActiveTheme().

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: travkin79

com.microsoft.copilot.eclipse.ui/META-INF/MANIFEST.MF

@@ -71,5 +71,6 @@ Require-Bundle: com.microsoft.copilot.eclipse.core;bundle-version="0.19.0",
  org.eclipse.jdt.debug;resolution:=optional,
  org.commonmark;bundle-version="0.24.0",
  org.commonmark-gfm-tables;bundle-version="0.24.0",
- org.commonmark-task-list-items;bundle-version="0.24.0"
+ org.commonmark-task-list-items;bundle-version="0.24.0",
  org.commonmark-gfm-strikethrough;bundle-version="0.24.0",
+ org.eclipse.e4.ui.css.swt.theme;bundle-version="0.14.500"

com.microsoft.copilot.eclipse.ui/src/com/microsoft/copilot/eclipse/ui/utils/UiUtils.java

@@ -35,7 +35,8 @@
 import org.eclipse.core.filesystem.IFileStore;
 import org.eclipse.core.resources.IFile;
 import org.eclipse.core.resources.IResource;
-import org.eclipse.core.runtime.preferences.InstanceScope;
+import org.eclipse.e4.ui.css.swt.theme.ITheme;
+import org.eclipse.e4.ui.css.swt.theme.IThemeEngine;
 import org.eclipse.e4.ui.model.application.ui.basic.MPart;
 import org.eclipse.e4.ui.services.IStylingEngine;
 import org.eclipse.e4.ui.workbench.modeling.EPartService;
@@ -88,7 +89,6 @@
 import org.eclipse.ui.part.IShowInTarget;
 import org.eclipse.ui.part.ShowInContext;
 import org.eclipse.ui.texteditor.ITextEditor;
-import org.osgi.service.prefs.Preferences;
 
 import com.microsoft.copilot.eclipse.core.CopilotCore;
 import com.microsoft.copilot.eclipse.core.utils.PlatformUtils;
@@ -505,10 +505,17 @@ public static void applyChatFont(Control control) {
    * @return true if dark theme is active, false otherwise
    */
   public static boolean isDarkTheme() {
-    Preferences preferences = InstanceScope.INSTANCE.getNode("org.eclipse.e4.ui.css.swt.theme");
-    String themeCssUri = preferences.get("themeid", "");
-    if (themeCssUri.toLowerCase().contains("dark")) {
-      return true;
+    IStylingEngine stylingEngine = PlatformUI.getWorkbench().getService(IStylingEngine.class);
+    if (stylingEngine != null) {
+      IThemeEngine themeEngine =
+          (IThemeEngine) PlatformUI.getWorkbench().getService(IThemeEngine.class);
+      if (themeEngine != null) {
+        ITheme activeTheme = themeEngine.getActiveTheme();
+        if (activeTheme != null) {
+          String themeId = activeTheme.getId();
+          return themeId.toLowerCase().contains("dark");
+        }
+      }
     }
     return false;
   }