The way a Drupal site looks comes from which "theme" is selected. There are a handful of themes that are included with the default installation, and many more that can be installed. (Apparently there are potential security risks associated with poorly coded themes; I will need to look into this further before recommending where and how people can get additional themes.)
If you are logged in as an administrative user, you can select "Administer" from the sidebar, then select "Site Building" then "Themes". You will see a list of available themes, which can be selected and adjusted via the "configure" button.
You can set an overall default theme for your site, but administrative users can select a different theme. When logged in as an administrative user, select My Account then click the edit tab. In the Account Setting section, there is a header for Theme Configuration. Expand that to see a list of enabled theme. Click in the "select" radio button alongside the theme you want to set that theme.
My Scrap Exchange site is set to have a default theme of Chameleon, but for Admin, I've selected the Garland theme, so when I'm working on the site as an administrator and looking at it in another window as a user, it's clear which is Admin and which the user.