Add final artwork and correctly read colours.

Merged username-removed-25042 requested to merge pserwylo/fdroidclient:category-tweaks into master

It was pointed out in #1039 (closed) that some category backgrounds were missing. We indeed don't have artwork for all backgrounds, but I did neglect to add the Games category background that Carrie created. In addition, I also noticed that I wasn't using the colours that Carrie suggested, so this fixes the code which asks for the category background colours.

Fixes #1039 (closed).