On this theme, you can use WP Facebook Login or WordPress Social Login to Sign to web, just need to use plugin WP Facebook Login, follow these steps below:
1. Install WP Facebook Login or WordPress Social Login plugin .
2. Go to https://developers.facebook.com/apps/
3. Create the new api(How to create API?).
4. Copy code and insert it to your site.
5. Facebook Avatar, this option allow User can changed their avatar or not if they login with Facebook.