Bug 31919: Add backgground color to hovered items in header dropdowns

Bug 31919: Add backgground color to hovered items in header dropdowns

This alternate patch is different in a few ways:

- Add a hover background color only to items in dropdown menus.
- Use an SCSS variable for the background color
- Update the specificity of the hover CSS to more fully override
  Bootstrap's default.

To test, apply the patch and rebuild the staff interface CSS.

- On any page in the staff interface, test the behavior of links in the
  nav header.
- The always-visible links (Circulation, Patrons, etc) should keep the
  same style as before: Underline on hover.
- In dropdown menus like "More" and the user menu in the upper right
  corner, hovered links should have a green background.

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>