Hi,
The file to modify the popup styling is
/wp-content/plugins/ajax-login-and-registration-modal-popup/skins/default/skin.css
Some style could be also edited in the
/wp-content/plugins/ajax-login-and-registration-modal-popup/assets/lrm-core-compiled.css
But please note, after the plugin update all your changes will be lost, the better way to overrride styles in your child theme or using Custom CSS styles in the WP Customizer: https://www.wpbeginner.com/plugins/how-to-easily-add-custom-css-to-your-wordpress-site/
Also in the Pro version you could customize most of these things: https://docs.maxim-kaminsky.com/lrm/kb/customizing-skins-colors-pro-only/
Max
