[totaltec]

I'm sure you just reference the id in your CSS. After you make the change described above, you can add to any of your included style sheets, referencing #prefix_{$catid}

{$catid} is going to be replaced when the template is compiled, if your first category has a $catid of 101 and you want it to be blue, then in your CSS:
#prefix_101 {
background-color: blue;
}

Or something along those lines.