private static List<ThemeData> getAvailableThemes() {
        synchronized (availableThemesMutex) {
            if (!availableThemesInitialized) {
                //async scan not finished -> rescan synchronized and GUI-blocking!
                recalculateAvailableThemes();
            }
            //make a copy to be thread-safe
            return new ArrayList<>(availableThemes);
        }
    }
recalculate available themes out of the currently active folder
main/src/main/java/cgeo/geocaching/unifiedmap/mapsforge/MapsforgeThemeHelper.java