private static void addSystemUiVisibilityFlag(Activity activity, int flag) {
        View root = activity.getWindow().getDecorView().getRootView();
        int visibility = root.getSystemUiVisibility();
        visibility |= flag;
        root.setSystemUiVisibility(visibility);
    }
Sets navigation bar color. Makes the icons dark if necessary
customtabs/src/android/support/customtabs/trusted/Utils.java