private static void recalculateAvailableThemes() {

        final List<ThemeData> newAvailableThemes = new ArrayList<>();
        addAvailableThemes(isThemeSynchronizationActive() ? Folder.fromFile(MAP_THEMES_INTERNAL_FOLDER) : MAP_THEMES_FOLDER.getFolder(), newAvailableThemes, "", 0);

        Collections.sort(newAvailableThemes, (t1, t2) -> TextUtils.COLLATOR.compare(t1.userDisplayableName, t2.userDisplayableName));

        synchronized (availableThemesMutex) {
            availableThemes.clear();
            availableThemes.addAll(newAvailableThemes);
            availableThemesInitialized = true;
        }

        synchronized (cachedZipMutex) {
            cachedZipProvider = null;
            cachedZipProviderFilename = null;
        }
    }
recalculate available themes out of the currently active folder
main/src/main/java/cgeo/geocaching/unifiedmap/mapsforge/MapsforgeThemeHelper.java