How to use “Additional CSS” on your WordPress login page

The solution is remarkably simple; just add one line of code to your theme’s functions.php file (see here if you don’t know how).

add_action('login_head', 'wp_custom_css_cb');

That’s it!

WordPress allows you to tweak and customize your theme using extra CSS from the admin panel, Appearance –> Themes –> Customize –> Additional CSS

Anything you add here will be applied site-wide. Everywhere, except the login page, unfortunately. Using this line of code fixes this and lets you have a streamline visual experience across your entire site.