I just used your code on a demo site and it works fine; the background color changes to blue.
It should work for you, too. Did you add any other custom css styles that might affect your code?
I attached a screenshot how should your “Custom CSS” box should look like after you add the coe.
If you want, you can create a new user in wordpress, and give me username and password so I can check it out.
Be sure that you check “Set as private reply” at the bottom when you send the message.