Change hover of bookmark bar items

[cndouglas]

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Ran git rebase -i to squash commits (if needed).

Test Plan:

1. Open a new Brave window.
2. Make sure the bookmarks toolbar is visible (Menubar > Bookmarks > Bookmarks Toolbar).
3. Add a new bookmark to the bookmarks toolbar, if necessary.
4. Move the mouse pointer (hover) over the bookmark item.
5. Make sure it has a white background with a light gray shadow (like the back and reload buttons).

Fixes #5665.

This matches the hover appearance of the toolbar buttons. Feel free to close if a different appearance is desired.

[bbondy]

@bradleyrichter pls do a UI review

[bradleyrichter]

I added 3 more changes to this but messed up the commit somehow?

23efd25

cc: @bbondy @liunkae

[bradleyrichter]

If we can roll these together, it's good to go! Looks great...

[bsclifton]

cherry-picked Brad's commit into here; looks good to me 😄

Here's a demo screenshot of what it looks like with this PR:

Versus what it looks like with the current release (can't see the hover personally)

[cndouglas]

Thanks @bradleyrichter and @bsclifton. Looks great!