That’s pretty fine and it’s the proper way to add CSS to login page. But you can also change login page CSS by below code-
function the_dramatist_custom_login_css() {
echo '<style type="text/css"> //Write your css here </style>';
}
add_action('login_head', 'the_dramatist_custom_login_css');
This actually prints CSS code inline in the login page. And for admin CSS your way is correct.