[themes] Apply OS-specific UI scale factor to em values

src/core/qgsapplication.cpp

@@ -759,6 +759,23 @@ void QgsApplication::setUITheme( const QString &themeName )
   }
   file.close();
 
+  if ( Qgis::UI_SCALE_FACTOR != 1.0 )
+  {
+    // apply OS-specific UI scale factor to stylesheet's em values
+    int index = 0;
+    QRegularExpression regex( QStringLiteral( "(?<=[\\s:])([0-9\\.]+)(?=em)" ) );
+    QRegularExpressionMatch match = regex.match( styledata, index );
+    while ( match.hasMatch() )
+    {
+      index = match.capturedStart();
+      styledata.remove( index, match.captured( 0 ).length() );
+      QString number = QString::number( match.captured( 0 ).toDouble() * Qgis::UI_SCALE_FACTOR );
+      styledata.insert( index, number );
+      index += number.length();
+      match = regex.match( styledata, index );
+    }
+  }
+
   qApp->setStyleSheet( styledata );
 
   QFile palettefile( path + "/palette.txt" );