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

[Wentao Zhang via llvm-commits]

whentojump wrote:

I believe "cyan" specifically refers to `#00ffff`? The color is also used in `llvm-cov`'s text output for expansions.
Maybe let's not change either (font) color or background-color yet.



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