Removing this old background color solves the problem of the bottom of short pages (like `/admin`'s login page) being white. The background was being set to black, which would be inverted, so it'd appear white. Since the `filter:` css has [~97% support](https://caniuse.com/?search=filter), I think that this change should be made. Tested on latest versions of Chrome (mac and iOS), Firefox, and Safari (mac and iOS).
