Misc fixes to support having a CiviCRM menu of different colours and backwards compatibility

Misc fixes to support having a CiviCRM menu of different colours, and makes sure we provide a seamless upgrade for those using the old behaviour.

I had to do a bit of a CSS hack so that the hovering of menu items works like before, because otherwise it's a bit ugly (too dark).

With a white menu:

Black menu:

Vanilla CiviCRM using a white menu:

cc @jaapjansma Apologies if this complicates your upgrades. If you use another colour for the CiviCRM menu, you will need to reset the setting after the upgrade.