general background color on pages

hello,

i am using ablock theme.

if anyone could please guide me on how do i set a background color on pages where i have my content?
a general background color, say a gray color. so that every pages and every content has an automatic gray background color. at the moment im am using the table background html code everytime i create new content or page or story. i know this is possible but i dont know which line or code in css style to modify.