WET Theme: Footer, menu and background

@thomasgohard I changed the colours for the footer, menu and background. I will work on the next PRs. Let me know if I'm doing something wrong. It has been a while since I made my first pull request. 


The `<footer>` element can be used in any sectioning element; a page may have multiple. So changing the selector from the ID `#wb-info` to `footer` broadens the scope of the CSS rule; it will apply to all footer elements, not just the one with ID `#wb-info` (the site footer).

As these changes are supposed to apply to the theme footer, menu and background, I noted that the rule now applies to more than the site footer (`#wb-info`) and that this may be an error.