Can GTK do all of this?

The new seek circle looks good, so did the last one. The checkboxes in settings need to stand out more, too hard to see. And you should gray out any control buttons that aren't active with settings open, like the playlist button. Unless you want to use it, in addition to the settings button, to close settings.

That red close button has bugged me since the first mockup though. It stands out too much because it isn't blue/black/white like the rest of the window. Maybe darkening the red would help, or changing it to a blue X like diablo5.