WIP: Inverting grey white for focus

What does this MR do?

Inverts white and grey colors for nav and content bodies

Are there points in the code the reviewer needs to double check?

WIP - this is dirty code

Why was this MR needed?

I think it doesn't make sense to have grey elements be on top of white elements. Also the unending white of a page can give a little un easy feeling due to vertigo.. it unfocuses the user on the content. This is to give a feel for how it might work

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

cc: @tauriedavis @cperessini @pedroms @annabeldunstone (launch it in the gdk and change some things.. to get a feel)