Change navigation menu background colour

Hello!
So I just activated the ‘Firstyme’ theme for my website but there’s one thing I can’t figure out – how do I change the background colour of the navigation bar/menu thing? It’s on the far right, there’s only ‘Sample Page’ there at the moment, but as you can see, it’s rather difficult to spot…I’d like the background to be white, like the background of my main posts and widgets on the left-hand side. How do I do this?