Lead Developer
Honestly, if the plugin is adding !important to everything it generates, I suggest finding a different one. Using !important should only be used in certain cases, and rarely at that.
Are you just trying to add background images to your navigation? Have you taken a look at our Backgrounds add-on?
Documentation: http://docs.generatepress.com/
Adding CSS: http://docs.generatepress.com/article/adding-css/
Ongoing Development: https://generatepress.com/ongoing-development