Drupal Themes
There are many themes for Drupal and there are also modules that can further customise a theme such as the Color module included in Drupal core. Themes can be found at: http://drupal.org/project/Themes
Again, you will need to make sure you are using the correct version.
Some themes such as Zen and Genesis are so called 'Starter Themes'. They are great for teaching you how to create your own theme and are designed to be customised themselves using 'Subthemes'. This is the place to start if you really want to take control over how your site looks. More on that later.
Have a browse and download a theme you like. Unpack it and then upload it to /sites/all/themes/ on your site. You may need to create the 'themes' folder. The path to your theme files should then be /sites/all/themes/your-theme-name/
Go to Administer > Site building > Themes, then enable, set your theme as default and click 'Save configuration'.
All themes have configuration options such as changing the site logo and displaying post information which can be found under: Site building > Themes > Configure
Note that there are global settings as well as settings for individual themes. Some themes will also allow you to change the background colours!