[Theme: Unsail] How to add css and edit function.php

Hey So I am using the unsail theme and there are some custom things I would like to add to it but when I last edited the function.php file it made my entire site stop working. What I would like to do is change the resize of images that were set as featured image( currently the are too large for the frame), change the background color of certain sections of the page, and edit the portfolio section(i would like to create some space between each item displayed). The theme uses page builder by siteorigin.