The plugin allows you to use your own custom stylesheets.
In order to customize the style of the plugin, you can easily create a file called custom-login.css within the theme folder. The file will be automatically loaded from your theme instead of the plugin folder.
In this way, the style.css file contained within the plugin (/assets/css/) will be replaced with your stylesheet. Keep in mind to copy and paste the code included in this file if you need it.