For all Joomla 3+ templates built using the Zen Grid Framework v4 (any theme after October 2014) please refer to the Zen Grid Framework v4 documentation.
There are two primary ways that you can add your own modifications to your v2 Zen Grid Framework template. It is advised that any changes made to your template use these methods rather than editing the template css files directly as this makes it more challenging to update your template if there are changes to the theme.css files.
The user folder can be found in all zen grid framework version 2 templates by going to:
templates / [your template] / user /.
If you are using template compression these files will also be included into the compression array.
This is more of a legacy option used in version 2 of the framework because it was the primary method for implementing custom css in version 1 themes.
If you add the custom.css file to the templates / [your template] / css folder this file will automatically be loaded into your website.