The NS Members Only plugin only has a few settings for styling.
All you need to do is install the plugin and enable it in the plugin manager.
If enabled, your site will only be accessible by registered users that login.
To configure options such as allowing registration, forgot username and forgot password the plugin follows all of the core Joomla User Manager options which you can find documentation on how to configure those settings at https://docs.joomla.org/Help36:Components_Users_Configuration
You can easily set some of the colors and an image that will display for the login block in the plugin settings.
License key: This is the plugin license key that allows one-click updates through the Joomla installer. You can find on https://nswd.co next to your list of active subscriptions. If you have an active subscription and the key inserted you will be notified of updates as they are released.
Image: You can insert an image here that will display at the top of the login box.
Backdrop color: Here you can set the color of the login background.
Modal background color: Here you can set the background color of the login box.
Modal borders color: Here you can sent the border color of the login box.
If you need to adjust any other styling you can do that through CSS. The CSS file is located at: http://yourdomain.com/plugins/system/ns_members_login/assets/style.css