In Drupal, your website's theme is how the different visual elements that make up your website are combined and placed. It includes the arrangement of blocks in various regions, different layouts on different pages, the selection of colors, fonts and graphics.
In Drupal Gardens, the ThemeBuilder lets you choose and customize your site's look directly in your browser. The Drupal Gardens ThemeBuilder gives you easy to use, yet powerful, theming tools so that you can give your Drupal Gardens website its own gorgeous look in minutes. You can use the themes that it comes with as is, tweak them a bit, or fully customize them to create a unique look.
Your Drupal Gardens themes are fully compatible and exportable. Behind the scenes, Drupal Gardens combines your styling choices and any image files you upload into valid CSS, creating a fully-exportable and compatible Drupal theme that you can use on any Drupal 7 website.
Here is an excellent introduction to Drupal Gardens themes by one of Acquia's design and themeing ninjas, Senior Front End Engineer and CSS specialist, Jesse Beach.