Syntax highlighting to color the background instead of the text

[shot-codes]

Check for existing issues

• [X] Completed

Describe the feature

See the image below. Screenshot is from Tonsky's Alabster theme (https://github.com/tonsky/sublime-scheme-alabaster). I feel the motivation is very much in line with the ethos of Zed and I would love to see it in Zed.

If applicable, add mockups / screenshots to help present your vision of the feature