Edit CSS for wp-login.php

After initially connecting via a social network, the user is redirected back to the wp-login.php page. My registration form has extra fields that are too wide to be contained in the default login div and so I want to edit the CSS. I thought I could just do that via the login_head or login_enqueue_scripts hooks but it seems your script actually re-creates this page in the wsl_process_login_new_users_gateway_start function (wsl.users.gateway.php file). That function does not seem to include any external CSS file or offer an obvious way to do so. Any ideas how I can change this without changing the file (which would obviously be overwritten on the next update)?

https://wordpress.org/plugins/wordpress-social-login/