Thanks for trialling Sitefinity.
There are a number of resources for your to reference if you want to use themes. Here are some:
Designer's guide to custom themes
CSS Guide to Sitefinity
Finally here is a step by step guide to what you are trying to achieve.
Option 1 is a template for creating a theme.
Option 2 is merely the registration of a theme. Once you have created the theme you have to register it: http://www.sitefinity.com/documentation/documentationarticles/designer-s-guide/themes/registering-a-theme
I hope this helps you.
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Sitefinity CMS Ideas&Feedback Portal
and vote to affect the priority of the items