Simplify nav hovers

[janbrasna]

One-line summary

🚧 WIP

Significant changes and points to review

Accomplishing the same effect while being less fragile.
(Real typographic underline makes descenders more legible by avoiding them instead of crossing.)
Using defaults where possible.
Changing to relative distance to adapt better to type scaling.

Issue / Bugzilla link

Fixes #15850

Testing

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.28%. Comparing base (9db97b8) to head (840f2b2).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15855      +/-   ##
==========================================
+ Coverage   79.22%   79.28%   +0.06%     
==========================================
  Files         159      159              
  Lines        8287     8343      +56     
==========================================
+ Hits         6565     6615      +50     
- Misses       1722     1728       +6     

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