private static void setSelectedTheme(final ThemeData theme) {
        Settings.setSelectedMapRenderTheme(theme == null ? StringUtils.EMPTY : theme.id);
    }
Set a new map theme. The theme is evaluated against available themes and possibly corrected.
Next time a map viewer is opened, the theme will be evaluated and used if possible
main/src/main/java/cgeo/geocaching/maps/mapsforge/v6/RenderThemeHelper.java