The CSS files will be in the theme directory. The theme directory could be in one of several places, depending on which theme you're using.

The 3 built-in themes that come pre-installed with the system are here:
YOURSITE/concrete/themes
(note that the "Plain Yogurt" theme is in a folder called "default")

If you downloaded a theme from the marketplace, it will be in the YOURSITE/packages directory.

If you built your own theme, it's in YOURSITE/themes

If you want to modify one of the built-in themes, it's usually best to copy the theme folder out of the concrete/themes directory and place it in your top-level "themes" directory -- this way you can make changes without overwriting the core files (so if you mess something up and want to go back to the original, those files are still intact -- and also if you upgrade the Concrete5 system in the future, the upgrade won't overwrite your changes, which it would do if you directly modified the files in the concrete/themes directory).