We are using two custom css files (main.css, responsive.css). We have bootstrap.min.css file as a css framework and several plugins.
The first one (main.css) is the main custom css file which contains all of the specific styling for all the pages. The file is separated into sections like the following structure:
The second one is a media query file (responsive.css) to make the webpage fit for any devices. The structure of this file is as same as the main.css file displayed above.
If you would like to edit a specific section of the site, simply go to the main.css file and find the instructions at the top of the each section.