Maintenance and upkeep of panelset CSS

[gadenbuie]

A few improvements all around:

• Simplify how the active tab border bottom is handled
• Use CSS pseudo-private properties
• Inherit global CSS custom properties on :root rather than scoped to .panelset
• Use testthat 3rd edition snapshots

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (1ac9654) 51.46% compared to head (c4a2579) 52.16%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #192 +/- ## ========================================== + Coverage 51.46% 52.16% +0.69% ========================================== Files 20 20 Lines 783 784 +1 ========================================== + Hits 403 409 +6 + Misses 380 375 -5 ```

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