Only add option list widget styling for default theme

(This allows customized styling for non-default themes,
fixes white background on white text issue et cie)

src/app/qgisapp.cpp

@@ -755,7 +755,6 @@ QgisApp::QgisApp( QSplashScreen *splash, bool restorePlugins, bool skipVersionCh
   mStyleSheetBuilder = new QgisAppStyleSheet( this );
   connect( mStyleSheetBuilder, &QgisAppStyleSheet::appStyleSheetChanged,
            this, &QgisApp::setAppStyleSheet );
-  mStyleSheetBuilder->buildStyleSheet( mStyleSheetBuilder->defaultOptions() );
   endProfile();
 
   QWidget *centralWidget = this->centralWidget();
@@ -3234,6 +3233,7 @@ void QgisApp::setTheme( const QString &themeName )
 #endif
 #endif
 
+  mStyleSheetBuilder->buildStyleSheet( mStyleSheetBuilder->defaultOptions() );
   QgsApplication::setUITheme( theme );
 
   //QgsDebugMsg("Setting theme to \n" + themeName);

src/app/qgisappstylesheet.cpp

@@ -89,6 +89,7 @@ QMap<QString, QVariant> QgisAppStyleSheet::defaultOptions()
 
 void QgisAppStyleSheet::buildStyleSheet( const QMap<QString, QVariant> &opts )
 {
+  QgsSettings settings;
   QString ss;
 
   // QgisApp-wide font
@@ -122,24 +123,28 @@ void QgisAppStyleSheet::buildStyleSheet( const QMap<QString, QVariant> &opts )
 
   ss += QLatin1String( "QGroupBox{ font-weight: 600; }" );
 
-  //sidebar style
-  QString style = "QListWidget#mOptionsListWidget {"
-                  "    background-color: rgb(69, 69, 69, 0);"
-                  "    outline: 0;"
-                  "}"
-                  "QFrame#mOptionsListFrame {"
-                  "    background-color: rgb(69, 69, 69, 220);"
-                  "}"
-                  "QListWidget#mOptionsListWidget::item {"
-                  "    color: white;"
-                  "    padding: 3px;"
-                  "}"
-                  "QListWidget#mOptionsListWidget::item::selected {"
-                  "    color: black;"
-                  "    background-color:palette(Window);"
-                  "    padding-right: 0px;"
-                  "}";
-  ss += style;
+  QString themeName = settings.value( QStringLiteral( "UI/UITheme" ), "default" ).toString();
+  if ( themeName == QStringLiteral( "default" ) )
+  {
+    //sidebar style
+    QString style = "QListWidget#mOptionsListWidget {"
+                    "    background-color: rgb(69, 69, 69, 0);"
+                    "    outline: 0;"
+                    "}"
+                    "QFrame#mOptionsListFrame {"
+                    "    background-color: rgb(69, 69, 69, 220);"
+                    "}"
+                    "QListWidget#mOptionsListWidget::item {"
+                    "    color: white;"
+                    "    padding: 3px;"
+                    "}"
+                    "QListWidget#mOptionsListWidget::item::selected {"
+                    "    color: black;"
+                    "    background-color:palette(Window);"
+                    "    padding-right: 0px;"
+                    "}";
+    ss += style;
+  }
 
   // Fix selection color on losing focus (Windows)
   const QPalette palette = qApp->palette();