Thank you, the overriding worked. I love your theme but I still have problems with the radio buttons(among other icons): the right radio button textures are shown in the PopupMenu of a OptionButton, but the radio button icons are totally blurry, as if the texture filter is not set correctly. But I checked every single setting and explicitly set the filtering but it didn't help.
I have no more ideas left as to how to solve this, I feel like I tried everything. Maybe you have an idea?