You can create a theme - how you want your event site to look - by selecting the "Create new theme" link from the Event Site page, and you can edit existing themes by selecting the "[Edit]" link that follows each theme name on the same page.
Once you have done either of these things you will be presented with a very simple form, comprising a field for a name for the theme and a larger field labeled CSS.
CSS (Cascading Style Sheets) is a very simple and popular language used to present web pages, and you will need some knowledge of CSS if you want to make or edit themes. You needn't be a master by any means, and there is a CSS Tips help page designed specifically to help those who know nothing about the subject.
All you need to do is drop your CSS code into this box, and although you can write it directly, it is advisable to write it elsewhere, in a code or text editor first.
You will find that all of the themes you create will be available to any events you have in your account.