How can i change the default background from the default theme?


I’m using the default theme on RapidWeaver, and I’d like to change the default background for the entire theme or something, but how can I do that? I’m new to RapidWeaver so if someone could help me out with this that’d be great. I tried something in the CSS and my background turns out to be grey with 50% opacity.

I’m sorry if I’d place this in the wrong forum or something…