Customizing the colors?

[roracle]

My site has a dark theme, and I was wondering if there's a way to customize the colors of the panels, etc.
Here's some screenshots to show how it look now, and why it would be necessary to theme it.
I have the customizer, so maybe there's a way to add custom CSS to it so it looks right? I just wanted to find out if there's a "best practices" for ensuring it's themed correctly.

Thank you!