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

[Hana Dusíková via llvm-commits]

hanickadot wrote:

Interesting the line 65 with `url.clone()` is now block and before it was yellow only to end of line. Can't find the corresponded change in diff.

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