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

[Jan Patrick Lehr via llvm-commits]

jplehr wrote:

HI,
Can you please fix the test (and in the meantime revert if the fix takes longer)?
You should be able to reproduce the fail locally via `ninja check-llvm` (or `make check-llvm`, depending on your build tool).
Thank you!

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