How to change background on my Buddypress theme?

I have a buddypress theme (Frisco) for my website. I know html and css but in the style.css file and the other css document that the theme uses (green.css) I can’t seem to find the code to change so the background will not be white anymore.

Is there a way to change the background making a “custom.css” file? If so, how would I go about doing that so that my code will make changes to my site?

Thanks,
Kyle