General utility variables:
- $base_path: The base URL path of the Drupal installation. At the very least, this will always default to /.
- $css: An array of CSS files for the current page.
- $directory: The directory the theme is located in, e.g. themes/garland or themes/garland/minelli.
- $is_front: TRUE if the current page is the front page.
- $logged_in: TRUE if the user is registered and signed in.
- $is_admin: TRUE if the user has permission to access administration pages.