fix: studio dropdown hover styles

[KristinAoki]

JIRA Ticket: TNL-11205

Nav bar text disappears when focus and hover states are both true. Very interesting find! This will be true for all pages using the header from frontend-component-header. This does not happen when a user is only hovering over a button. This bug only appears when using the edx brand styles. This seems to be happening to all buttons that use the tertiary variant.

Testing

1. Using the tab key, tab over to any of the nav bar dropdowns.
2. Hover over the active dropdown with your mouse.
3. All text should remain visible with no button color change.
4. Hover over another button that is not focused.
5. The background of the button should be blue with white text.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (665653e) 66.96% compared to head (6235a92) 66.96%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #433   +/-   ##
=======================================
  Coverage   66.96%   66.96%           
=======================================
  Files          25       25           
  Lines         339      339           
  Branches       78       78           
=======================================
  Hits          227      227           
  Misses        110      110           
  Partials        2        2           

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