Add more style fields/theme settings for menu

[jasonnrosa]

From some feedback we got internally a lot of styles on the boilerplate's menu are hard coded/not available to be edited by a content creator (the menu uses rems which inherits font size from the body in theme settings right now so it kind of pulls in from theme settings but not in an intuitive way). I think it would make sense to be a little bit more prescriptive here (although we should still lean on the side of simple for boilerplate) so that we demonstrate best practices.

I think it could make sense to start with the menu font size + color options as a baseline to still keep things fairly simple but allow for a bit more flexibility in the theme editor the end user.