Set subsite menu colors to dark with light text

[willguv]

As many councils don't have dev help immediately to hand, we need the menu provided by subsite extras to be as useable as possible out of the box

This is what the menu currently looks like, which can't be used out of the box

We need to set as default:

• the menu background color to dark
• the text color to light

This seems to be the pattern used by most councils I've seen

We can provide controls for councils to set these colors at a later date