Improve experience when themes provide transparent status colors (#13996)

We shouldn't assume all themes will give us solid status color
backgrounds.

This change makes it so the status color renders on top of a normal
elevated surface background.

#### Before | After (Transparent status background color – Fixed)

![CleanShot 2024-07-09 at 10 50
17@2x](https://github.com/zed-industries/zed/assets/1714999/5f4b24c1-335a-4ed8-a1d0-f511e217e4a5)

![CleanShot 2024-07-09 at 10 50
31@2x](https://github.com/zed-industries/zed/assets/1714999/38c06533-bda5-4cfb-822a-ed5a9639fc33)

---

#### Before | After (Solid status background color – No change)

![CleanShot 2024-07-09 at 10 49
43@2x](https://github.com/zed-industries/zed/assets/1714999/bd60c807-a7bb-4f60-ab47-ddba17288e93)

![CleanShot 2024-07-09 at 10 49
58@2x](https://github.com/zed-industries/zed/assets/1714999/6ab27d60-5a77-448c-a23b-569b337f11e1)



Release Notes:

- Improved support for transparent status colors in themes.