You can add CSS to the Appearance → Custom Design → CSS editor in your blog dashboard.
Changing the background color of the sidebar in Mystique may take a little work. Here is a basic example to illustrate:
You can try it out by adding it to your Appearance → Custom Design → CSS editor and clicking the Preview button.
Change the color name to a color code of your choice:
If you make the change, note that the sidebar area is a block that may stop short when the main content area has a lot of content in it. Try adding a test post with a lot of content to see what I mean.
Also, this is another good resource you can check out if you are starting out with CSS: