Without using Easy Customization (which would be the easiest way to customize all colours and everything), and if you're using CSS, look for something like:
Just change the #ffffff's to the colours you want.
.thread0 and .thread1 alternate, so you can have one post one colour, and the next post in another, or just have both the same.
If you're not familiar with CSS, I'd say to go the Easy Customization route though
In Easy Customization, it's under "Alternate Thread BG Color #1" & 2.