A good enhancement to the IDE would be automatic reloading of external theme CSS files, so a restart of the IDE (Or I believe theme switch) wouldn't be required. Also, taking advantage of SCSS would be useful with automatic change detection and automatic compiling/applying would be a good and fairly simple addition.

With the use of an SCSS compiling library, a change detector could be inserted into the ThemeManager class upon changing the active theme. Change detection could be simply derived from the TextEditorManager class, though it would be better to make a separate class just for general file change watching with appropriate callbacks.