Set default colours for subsites extras

[markconroy]

Closes #550

Default is now:

1. Background colour of region: accent
2. Background colour of menu items: white
3. Colour of menu items text: accent
4. Border on menu items: white

(Basically, the opposite of what we had.)

=== Thanks to Big Blue Door for sponsoring my time to work on this.

[willguv]

Can you add a screenshot please @markconroy cheers. Thanks for looking at this so quickly

[markconroy]

@willguv screenshot added above just now

[finnlewis]

@willguv would like to discuss the menu items looking like buttons. Potentially want a slightly different approach for subsites extras menus.

[willguv]

@markconroy is it easy to make it look like this out of the box?

https://new.dumgal.gov.uk/gretna-registration-office/your-wedding-ceremony

[markconroy]

@willguv Some updates:

1. Default state - no background colour on menu links
2. Hover state - white background, with link underlined
3. Active trail state - white background, no underline, menu tab merges with white section of page below it

All the variables we created are still there so easy to have any style you want, and some new variables have been created to make this even more flexible.

Default:

Hover:

Active:

[willguv]

That's spot on, thanks @markconroy!