Use common-background code to enable variants of themes (eg. gruvbox contrast settings)

[neiljp]

Description of suggested feature

Following #1415, it is easier to change the background color for a theme in a central location, and distinguish that color from where it is used for another purpose.

Other than updating this manually, this may help support the variations that some themes possess.

One good example of this may be gruvbox, where there are three contrast options, which possibly correspond to different background colors in each of the dark and light themes:
https://github.com/morhetz/gruvbox?tab=readme-ov-file#contrast-options
(also see the bg0 variants of colors above that)

Implementing this may be as simple as supporting a list of background values to select from, instead of only one, and would certainly be preferable to individual themes for each contrast setting.