Hi bookerzm,

The CSS file path for the default Elemental theme is this:
concrete\themes\elemental\css\main.less

Editing this file directly requires an understanding of Less (http://lesscss.org/features/ ) and knowing that any changes made to it will be overridden when you update concrete5.

To prevent the CSS files from being overridden, you will need to make a copy of the theme.
http://www.concrete5.org/documentation/how-tos/designers/how-to-cop...

A simpler alternative to all of this is to add CSS to the Customize CSS section instead.
Toolbar > Page Settings (gear icon) > Design > Theme > Customize > Custom CSS gear icon button