Do not unregister themingParticipantChangeListener.

Fixes #27097

src/vs/workbench/services/themes/electron-browser/workbenchThemeService.ts

@@ -396,10 +396,6 @@ export class WorkbenchThemeService implements IWorkbenchThemeService {
 
 		themeId = validateThemeId(themeId); // migrate theme ids
 
-		if (this.themingParticipantChangeListener) {
-			this.themingParticipantChangeListener.dispose();
-			this.themingParticipantChangeListener = null;
-		}
 
 		return this.findThemeData(themeId, DEFAULT_THEME_ID).then(themeData => {
 			if (themeData) {
@@ -446,7 +442,9 @@ export class WorkbenchThemeService implements IWorkbenchThemeService {
 			$(this.container).addClass(newTheme.id);
 		}
 		this.currentColorTheme = newTheme;
-		this.themingParticipantChangeListener = themingRegistry.onThemingParticipantAdded(p => this.updateDynamicCSSRules(this.currentColorTheme));
+		if (!this.themingParticipantChangeListener) {
+			this.themingParticipantChangeListener = themingRegistry.onThemingParticipantAdded(p => this.updateDynamicCSSRules(this.currentColorTheme));
+		}
 
 		this.sendTelemetry(newTheme.id, newTheme.extensionData, 'color');