Separate theming for inline-diagnostics and diagnostics underlines

[carrascomj]

As a user, I'd like to have separate theming for diagnostic underlines and inline diagnostics messages.
I can implemented it myself if this is desired. I could not find it in the issues, but I am positive it was mentioned in the Inline Diagnostics PR.

Desired state

The diagnostics are separated from the text (dimmed) while the underlines are easy to spot. I guess the character separator also improves separation. Screenshot of my neovim setup:

Current state

If you dim the diagnostics, the underlines are not clear.

If you don't dim the diagnostics, the messages become difficult to differentiate from the code. Notice that the diagnostics messages are closer (2 characters) than the ones in neovim (5). I am not a UX expert, someone else should decide which one is better.