Does anyone know of a theme that completely removes all uses and aspects of the frontend. So the sole use of the site will be in the admin section. to use as a portal for something like a school or internal government uses.
There are three things (if i understood your needs correctly):
Adjust the admin menu items @MikeSchinkel Gist
wp_redirect();on login and offer only a login screen in your index.php template.
You could very easily create a theme that just acted as a landing page, with a link to the admin area.
To create a valid theme, all you need is a style.css with the appropriate mark up (see codex here) and an index.php. The index.php needs not contain any actual PHP, and you can create a holding page using just HTML.
You could also completely block the access to the front site with a .htaccess rule.