Enable a theme or disable a theme

The theme or appearance page at admin/appearance has two sections.  One shows the enabled themes.  The other shows the disabled themes.  From here you can enable and disable a theme and set a theme as the default theme.

The example above shows the location of the enabled and disabled themes.

To disable and enabled theme, thus turn off a theme, just click the disable link next to the theme.  Be advised, you are required to have at least one theme active for Drupal to be happy.

Before enabling a theme, you should understand the difference between an enabled theme and default theme.
There is nothing in Drupal stating that a theme is the default theme.  There is a link showing which enabled themes could be set as default.  Its weird.

The green arrow shows all the enabled themes.  The blue arrow shows the settings for the enabled theme.  The red arrow shows a link to set an enabled theme as default.

When you activate/enable a new theme, by clicking the enabled link in the disabled themes section, Drupal will refresh and a green box will confirm your new theme with a message similar to this, "<THEME> is now the default theme"

The red arrow shows you where to click to enable a theme.  The blue arrow shows you where to click to enable and set a theme to default.