Give core-toolbar a default background color

I'd like to add a default grey background color to core-toolbar so it's very clear to a new user that they have successfully added one to their page. Right now, core-toolbar is transparent, which can initially lead people to think that they've messed something up when they just get a blank screen. I realize this may mean going back to some of our demos which rely on the transparent behavior of core-toolbar and updating them to specifically not have a background, but I think/hope the trade-off is worth it for the increased feedback to new users.