Sometimes we are asked by clients if the Concrete5 login page can have the site theme applied to it. The answer is yes and it is relatively easy to do.
If you go onto your server in the root directory there is a folder named config. In that folder open the site_theme_paths.php file. After the defined('C5_EXECUTE') or die(_("Access Denied.")); line add the following code with 'mythemename' renamed to whatever the folder name that your theme lives in.
The end result? You should see your login page with your Concrete5 theme applied to it.