Yes the easiest way is to follow the steps in the Themes documentation topic specifically under the "Customizing Specific Colors in a Theme" section. There are a lot of properties on RibbonColorScheme that deal specifically with RibbonWindow so by changing any of those and then applying the brushes again (also explained in that topic), you can alter the look of the RibbonWindow.
Alternatively you can override our default brushes that we use for Window. The color modification described above is better because it keeps all the gradients in tact while allowing you to change any color. But if you do need a more customized look, the brush way is how you would go and we can provide a list of the built-in brushes related to Window if you need.