fix: DH-18562: Deephaven picker hover blends with non-hovered styling in dark mode

[gzh2003]

DH-18562: Fixes issue with picker styling when using dark mode. Previously the background did not serve as a good indicator when hovering over items.

I checked in with @dsmmcken to confirm the styling changes. I also tested the picker component in both the style guide and theme selector under settings.

Hovered Styling (Before)

Hovered Styling (After)

[github-actions]

CLA Assistant Lite bot All contributors have signed the CLA ✍️ ✅

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 47.06%. Comparing base (6d65594) to head (0174eb0).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2433   +/-   ##
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  Coverage   47.06%   47.06%           
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  Files         714      714           
  Lines       39468    39468           
  Branches    10063    10063           
=======================================
  Hits        18576    18576           
  Misses      20881    20881           
  Partials       11       11           

Flag	Coverage Δ
unit	47.06% <ø> (ø)

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[gzh2003]

I have read the CLA Document and I hereby sign the CLA

[mofojed]

I assume you checked with @dsmmcken about which colour variable to use.

In the PR description, I encourage including details about how you tested a particular ticket (e.g. "Checked using the theme selector under settings") so it's clear what has been tested (for both the current reviewer, and yourself or any other developer who may come across this ticket in the future); and for a ticket like this which is a small visual change, a screenshot of before/after is also good.

Looks great, great work!