duarteixeira

STEPS:

1 - If the theme was not developd by you, first create a child theme (How to create a child theme? Read at https://codex.wordpress.org/Child_Themes) or else modifications will be lost when updating your theme.
2 - Add the following CSS file in your theme/child theme (better if you create a folder to add CSS there).
3 - Add the following snippet in your functions.php file:

function tpw_enqueue_custom_admin_style() {
        echo '<link rel="stylesheet" type="text/css" href="' . get_stylesheet_directory_uri() . '/css/admin/custom-login-styles.css" />';
}