Enabling and Setting a Default Theme
In a fresh installation of Drupal 7, the default theme (Bartik) appears at the top of the Appearance page, followed by other enabled and disabled themes (see Figure 15–5). What is a default theme? Simply enabling a theme is not enough with Drupal. Setting a theme as the default is what makes it the front-end theme (the theme your site visitors will see).
Enabling a theme without setting it as the default is useful when you want your site to utilize multiple themes at once. This setting is typically more useful when used in conjunction with contributed modules. An example of this is the SwitchTheme module, which allows users to change the site theme by selecting the name of a theme from a list that is populated with all enabled themes.