Support theme.images.additional_backgrounds

[FOREVEREALIZE]

Hiya!

I came to Zen from Firefox + Sidebery and one thing I really liked was that I could use this theme, especially with the starry background. However, Zen doesn't seem to support the theme.images.additional_backgrounds property that makes this possible. While MDN does call out this is an experimental feature in Firefox, it would be nice to see this implemented.

Perhaps supporting the additional_backgrounds system in its entirety right now is too much work so, instead, a "bandaid" solution of some kind could be implemented, like letting users pick a single background for the Sidebar + URL Bar, instead of the complex layering that additional_backgrounds allows users to make.