[llvm] [llvm-cov] Add HTML dark theme support (PR #93080)

[via llvm-commits]

EdJoPaTo wrote:

With the Firefox Dev Tools its relatively simple to test stylesheets. Copy the new ones and replace the style.css in the Dev Tools. That way the given styles can be previewed with the given output.

I spontaneously picked a personal Rust project of mine which hopefully includes a bunch of different styling to be viewed here.

The css_sort.rs file is hovered, Line 65 is selected (#L65 at the end of the URL)

<details>
<summary>Overview</summary>

## Before
![Screenshot from 2024-06-05 12-02-18](https://github.com/llvm/llvm-project/assets/7953011/dbfba88b-181b-4c60-8fcd-afc8d24eeb3a)

## After (bright)
![Screenshot from 2024-06-05 12-10-21](https://github.com/llvm/llvm-project/assets/7953011/eac1afc6-3b25-4ab7-bba5-71b4edef29f6)

## After (dark)
![Screenshot from 2024-06-05 12-10-15](https://github.com/llvm/llvm-project/assets/7953011/a43ec260-c7e6-4513-8b02-554a300ee30d)

</details>

<details>
<summary>Code view</summary>

## Before
![Screenshot from 2024-06-05 12-17-16](https://github.com/llvm/llvm-project/assets/7953011/9afe07a7-65dd-4083-86d9-bc6c620a04dc)
## After (bright)
![Screenshot from 2024-06-05 12-04-07](https://github.com/llvm/llvm-project/assets/7953011/a4580a6c-7ccc-4c8e-bb4e-de78fbc7c287)
## After (dark)
![Screenshot from 2024-06-05 12-04-17](https://github.com/llvm/llvm-project/assets/7953011/3468fe15-bbbe-4a98-81d5-7a2940c3e77a)

</details>


https://github.com/llvm/llvm-project/pull/93080