Update css such that PDF Export changes background color

Hey there,
I am using a lot of callout boxes with admonition plugin in a dark theme. I want to export my notes with these callout boxes in their color with my background color in my theme (which is #363636). I read that a possible solution is to create a css file with the background changed.

I am unfamiliar with how to set it up in obsidian, such that the print gets exported as I desire it to be. Help would be greatly appreciated.