Well, in terms of sheet development, there's not much you can do for the D&D 5th Edition by Roll20 Sheet, since the code is proprietary. Users don't have access to it. Unless you were messing around with CSS code in an extension like Stylus, I don't think this is anything you've done. I would suggest deleting cache and cookies from your browser, since it may be using cached light mode images while in dark mode. You could also test by running in a private browsing window. If the sheet looks fine there, it is almost certainly cached data or the action of a browser extension.