Lead Developer
As it looks like you’re using GP 1.3.15 (you should update), replace your functions.php file with this one: https://themes.svn.wordpress.org/generatepress/1.3.15/functions.php
Documentation: http://docs.generatepress.com/
Adding CSS: http://docs.generatepress.com/article/adding-css/
Ongoing Development: https://generatepress.com/ongoing-development