Symbolic suffix was added twice for icons media-skip-backward, media-playback-start, media-playback-stop, and media-skip-forward

[german-rios-gonzalez]

The file "player.js" use the class PlayerButton from the file "widget.js" in this way (note the use of the suffix -symbolic):

    this._prevButton = new Widget.PlayerButton('media-skip-backward-symbolic',
        Lang.bind(this, function () { this._mediaServerPlayer.PreviousRemote(); }));
    this._playButton = new Widget.PlayerButton('media-playback-start-symbolic',
        Lang.bind(this, function () { this._mediaServerPlayer.PlayPauseRemote(); }));
    this._stopButton = new Widget.PlayerButton('media-playback-stop-symbolic',
        Lang.bind(this, function () { this._mediaServerPlayer.StopRemote(); }));
    this._stopButton.hide();
    this._nextButton = new Widget.PlayerButton('media-skip-forward-symbolic',
        Lang.bind(this, function () { this._mediaServerPlayer.NextRemote(); }));

And the class PlayerButton add again the -symbolic suffix here:

setIcon: function(icon) {
this.icon.icon_name = icon + '-symbolic';

The extension works because the especial GTK+ fallback use of symbolic icons, for example it looks for media-skip-backward-symbolic-symbolic, and not finding it, removes the -symbolic suffix, and because it can find the now media-skip-backward-symbolic you didn't notice this before.