themes: Make background colors partly transparent by default (#24151)

Certain themes define the `created` and `deleted` status colors, but not
`created_background` and `deleted_background`. Previously, Zed would use
`created` and `deleted` colors, and apply a hard-coded opacity change,
but *not* use `created_background` and `deleted_background`, but that
behavior was inadvertently changed in
https://github.com/zed-industries/zed/pull/22994.

This PR restores the old behavior as a fallback. If a theme defines a
status color, but not the corresponding background color, we'll use a
75% transparent version of the foreground color as a fallback.

Release Notes:

- Fixed an issue in certain themes where diffs would render with the
wrong red and green colors for deletions and insertions.