how to change the background in easy bloging mode

I am using the easy bloging pluging and i'd like to make some changes in the background.
I wanted to change the color to #primary_left ( the menu left) but i couldnt get it.
I've tried in the custom admin css included in ultimate branding, and in another external plugings, but it didnt work.
Could you tell where do i have to place this css code??