matrix theme - change background color from grey to black - where???

hello professionals ,

i have a quesion about the background color, because i read all the posts (with the search function) about background color editing.
i tried the most of the offered solutions, but nothing works, in real, the editing i made in the theme.css file: /galerie/themes/matrix/ doesn't make any difference...

here is my gallery:
http://www.tattoo-sven.de/galerie/main.php

in which file i have to do the changes, and which changes? i used the matrix theme for all albums and the colorpack "schwarz" black for all albums, but it is still grey...
i don't know what i can do and now i'm still nerved about that and my stupidness...

please be so kindly and help me, theank you very much for your help...

greetz, coucie