Change the background color of the header

Hi ! I'm working on a new child theme of Catch Box for habitation.ch. The Catch-box theme is already running for this site, but they want a new look.Habitation got also a magazine, and they will change they design: the logo will stay in white and the background will change regularly. I got a logo with a transparent background. My idea is to apply a color background for the header section. How can I simply do this? Thanks for your support (hum... I presume I will get some more questions these next days...). Have a nice week-end! Regards, Marc