I have successfully (after much faffing!) themed a OptionButton and its PopupMenu (Love godot, but this could be documented better ) - however I still have a grey background on the popup I cannot shift.
I cant find any setting in the Theme to get rid of this.
try comparing the fps difference between custom made window/popup vs godot’s built in popup menu. it will tell a lot
by i meant custom made is not by using Window’s type node as its base. just a simple Control