So, I was using "Firefox Color" to design my own theme. All the block colours seem to work. However, there's an option for custom background, which the preview within the creator app shows nicely. When trying one image type, it notified me that it wasn't compatible and to choose another. So the one I chose should therefore be compatible and therefore would assume it should work. Yet, when the theme is applied, the custom background isn't there, just the block colour. Perhaps the block background colour is overriding the custom background? If so, though, I have no idea how to stop it from doing that so that I can actually see my custom one. Plz halp!