Lead Developer
Looking through all of our add-ons pages and videos is a good way to get an understanding of how you can use the add-ons: https://generatepress.com/knowledgebase/
For CSS, it’s very tough to teach through a forum – most of it is memorizing values etc.. There’s plenty of resources out there though. Codecademy.com is a good one for starters.
It’s also good to have a tool like Inspect Element (Chrome) or Firebug (Firefox) which allows you to play with the HTML/CSS in your browser and see the changes take place.