includes/templates/classic/css/stylesheet.css

That is the CSS file being used. If you are going to continue using the 'default' theme then you would edit that file. Otherwise, you need to use the 'Template Overrides' system which is a whole new ballgame.

Much information is available on the overrides system but does take some time to wrap your head around. Use the Tutorials/FAQ link at the top of this page, to learn more -- about the overrides or most anything to do with using Zencart.

And welcome to the Forum.