Changing Column Widths

Hi there,
I have a theme that is taking CSS instructions from a file called mobile.css In there, all column widths are specified eg: onecol, twocol etc.

My sidebar is using the fourcol code. Unfortunately so does my footer so when I try to give my sidebar a different background colour, it also applies to the footer.

Does anyone have any idea how I might change this, the author of the theme is no help whatsoever and I have been trying for days now. Its the only thing holding up the site from being launched!

PLEASE HELP!!

Thanks in advance,

Jenny