You need to change two parts of the CSS, the DIV body#bd background colour, and the DIV #gk-bg background image.
Open the file
template/gk_elvesocial/css/template.css and edit line 19 and change the colour (if you want black, it'll change from #ffffff to #000000)
Then upload your new background into the template/gk_elvesocial/images folder and change line 21 to point to your new background file in the same template.css file.
A brilliant tool to work out this kind of thing is Firebug for Firefox.
In Firefox, go to
Tools>Add-ons and search for Firebug, and install it. Use it to find the CSS element you need to change and on which CSS file you need to change it on.
No web designer should be without it.
Hope this helps!