First and most important CSS file called style.css contains all of the specific stylings for the page. The file is separated into sections using:

/* Structure */
some code

/* WordPress Core */
some code

/* 404 error */
some code

etc, etc.

If you would like to edit a specific section of the site, simply find the appropriate label in the CSS file, and then scroll down until you find the appropriate style that needs to be edited.