Add CSS variable for the results background

[bglw]

The Modular UI looks interesting, but it would be really nice if there was a CSS var that was exposed to allow styling of the background of the results area. I have the search box on every page of my site and when the visitor scrolls, it remains stuck at the top of the window. But when I do a search after such scrolling, the results are on a transparent background. It would seem that having a --pagefind-ui-results-background variable would be a welcome addition to the main repo. Until then, I'll keep my eye on the development of the Modular UI and remove the sticky page search box. Thanks for listening...and thanks for an incredible search package!

Originally posted by @bobmonsour in https://github.com/CloudCannon/pagefind/discussions/491#discussioncomment-8362735