How do I create more background options?

I am just starting out with the sporter template, having previously used Icki.

I like the way the template gives different options for displaying backgrounds by having the "red" and "brown" links at the bottom of the page, which change the background image.

I would like to expand on this with a few more options, so that if someone is a fan of boxing, or auto racing, or whatever, I can have an appropriate background available for them to choose. I don't really need to change the css links or anything, just the image. In other words, I can use all the same settings as the "red" option, for example, maybe just copying and pasting everything, and then changing the file name of the image that is loaded up.

Anyway, could someone please enlighten me as to how I do this? Is this something I just copy and paste into css, or do I need to create a new css file for each one? I'm coming into this fresh and really have no idea, so any help will be greatly appreciated.