Thank you very much for the advice, DaveBax! I gave that a shot. Active Window and Inactive Window have nothing, while Active Window (Glass) and Inactive Window (Glass) have images/code. But they both have the exact same code in them:
Removing this from Inactive Window (Glass) has no effect on the issue - the background color still changes. VENOM\bg.tga is a tiny 1x1 black image, and the color that the inactive window is changing to is white.