I need help in turning the white part of my profile (the one that shows every info) into black

Im not an expert on CSS so I need help in figuring out on how to do it.

If you dont know what I mean, when you go to my profile, you can see the main hub with all the info but its background is slightly transparent. That's what I mean. I wanna change its color and I don't know how. Any help is greatly appreciated.