Hi Tintapixel,
There are a couple of options.
You can pick the predefined theme that is closest to what you want, and then use CSS in your theme’s style.css file to override the styles of that theme.
Or you can use the jQuery UI theme roller to generate a css file. I’m working on a tutorial to better explain this, but essentially once you’ve generated your custom theme, you just have to pull out the css file and images and put them somewhere in your own theme, then put the url to that css file in the “Use your own theme” field on the Event Geek options page.
Hope that helps.