WordPress has a wide range of options to change the look and feel of your WordPress setup. One that most people overlook is the login box, accessible at http://yourdomain.com/wp-admin/.

Using the code below, you will be able to apply a custom CSS stylesheet to the login page.

Firstly create your stylesheet. You could name the file 'custom-login.css' and save it within your theme folder with your standard theme stylesheet.

You will need to add code into your functions.php so WordPress will know to load your custom-login.css document.

Once done, you should now see your CSS stylesheet applied when accessing the login page. If you need any more help, there is a great article on the WordPress Codex here.