You can change the theme used to display the site in the following ways:
- Use the theme switch dropdown selector
- Find a theme using the theme gallery and click the preview link
With some themes, it is possible to change the colour and other aspects of the theme. On these themes, there will be a link in the messages area to allow you to edit the theme settings.
If you find a theme you like, and want to show someone else, just copy the preview link for that theme and send it to them by email.
The Drupal 7 theme gallery is built by Ruby Design. We create bespoke websites, building custom Drupal themes for our clients. We wanted to offer an additional service for clients on a tight budget, whereby we would build a Drupal site using some of the community provided themes.