twentytwelve content background colour

Hi i would like to change the “content” background colour – the background, header and footer should be black and content white, but when i change the “content” colour what appears are small white squares around the articles? – how do i change the area between the header and footer to white. And id possible take out the shadow that would be part of whet i presume is the wrapper, but doesn’t seem to be

site is below
https://bunddesign.com/pdm/photographer-login/

Is there also way to add a 3 column widgets to the footer?
Thanks in advance
Melanie