Are you using a light windows theme? In my brief testing earlier on, changing to light mode made 'dark skins' also have a light background in that section.
If you say some dark skin had a dark background when the windows mode was set to light, perhaps uu could share the name of such skins for investigation?
But no really, there isn't a MusicBee skin element that overrides that part of windows. (I should be careful and add "that I know of" :-)