function initColors() {
    col.artist = RGB(255, 255, 255);
    col.info_text = RGB(255, 255, 255);

    if (!pref.darkMode) {
        col.bg = RGB(205, 205, 205);
        col.menu_bg = RGB(58, 58, 58);
        col.progress_fill = RGB(235, 59, 70);
        col.progress_bar = RGB(125, 125, 125);
        col.now_playing = RGB(0, 0, 0); // tracknumber, title, and time
        col.aa_shadow = RGBA(000, 000, 000, 64);
    } else {
        col.bg = RGB(50, 54, 57);
        col.menu_bg = RGB(23, 23, 23);
        col.progress_fill = RGB(235, 59, 70);
        col.progress_bar = RGB(23, 22, 25);
        col.now_playing = RGB(255, 255, 255); // tracknumber, title, and time
        col.aa_shadow = RGBA(128, 128, 128, 54);
    }

    col.rating = RGB(255, 170, 032);
    col.mood = RGB(000, 128, 255);
    col.hotness = RGB(192, 192, 000);

    col.playcount = RGB(000, 153, 153);
    col.dark_grey = RGB(128, 128, 128);

    col.tl_added = RGB(15, 51, 65);
    col.tl_played = RGB(44, 66, 75);
    col.tl_unplayed = RGB(126, 173, 195);
    col.tl_play = RGB(255, 255, 255); // each individual play

    // ALBUM ART DISPLAY PROPERTIES
    col.aa_border = RGBA(060, 060, 060, 128);
}
initColors();

// inside onSettingsMenu (note I might have changed the index numbers... so you probably can't cut and paste this)

    _menu.AppendMenuItem(MF_STRING, 1, 'Use Dark Theme');
    _menu.CheckMenuItem(1, pref.darkMode);

// down below in the switch statement in the same method

        case 1:
            pref.darkMode = !pref.darkMode;
            initColors();
            albumart = null;
            loadFromCache = false;
            on_playback_new_track(fb.GetNowPlaying());
            break;

// inside globals.js in the pref.add_properties()

    darkMode: ['Use Dark Theme', true], // true: use a darker background

// I also made some changes in themes.js, mostly inside setTheme() although this code might be refactored from what you have

    if (!pref.darkMode) {
        col.progress_bar = rgb(125,125,125);
    } else {
        col.progress_bar = rgb(23, 22, 25);
    }

//etc...