Like always, there are ways but those are not appreciated if not forbidden. You want to change the style of your foobar-window [$WS_..] (style is meant as a parameter-term, not creative). Colors etc. are given by your VisualStyle (theme), this would be a systemwide change.
AFAIK (not entirely sure) by the terms of the foobar2000-SDK no code/plugin is allowed to modify the parent-window if it's not given by itsself, means only UI-plugins should be able to do so - if at all! Maybe PUI has such possibilities, who knows, I don't use stuff like that.
Whatever I suggest to let the things like they are, otherwise change your VS or use
AutoIt3 to do your treatments.
controllers: forbearance with possible inexact statements (SDK etc.)