Improve Contrast for onfocus Menu Items

Creating issue from #1924112-5: Make sure tour toolbar button has :focus styling when tabbed to.

With the present look of the menu bar it is hard to see where you are as there is just a thin outline & link provided around the focused tab button.

I agree that often the browser doesn't do a very good job providing a visible outline of items on focus.

I recall though that the argument against specifically setting a bigger outline with Drupal forms in D7 was that this was something best managed by the browser itself. That if outlines aren't sufficiently highlighted in FF or Chrome then they should be fixed there rather than in the CMS.

Something like a different background color would be great, but right now there is also an underline & an outline in my tests so not sure how critical this is.