Help with themeing the Admin Theme
Admin pages are not nodes, so you would need to use the page template rather than the node template. You can copy your theme's page.tpl.php file and rename it to page--admin.tpl.php to create a template just for all admin pages.  If you need these changes to extend for node edit pages, then you would need to also create a page--node--edit.tpl.php file. 

Another option is the ThemeKey module; it allows you to specify which theme to display based on path (i.e. you could change it for all admin/* paths) and/or based on user role (change theme for super users and low-level admins). You could choose a contributed admin theme to display on these conditions, or create a sub-theme of your front-end theme to display.