Hi chromahoen,
By default, there should be one ‘custom.css’ file in /templates/purity_iii/css folder. However, if you want to customize one theme in this template and don’t want to loose these custom work when updating template, you can customize any theme via: /templates/purity_iii/local folder.
We’ve recently implemented a new feature in new version of T3 framework so that customer can add custom work in a folder which can’t be affected when updating template. You can check out this documentation for more information: http://t3-framework.org/documentation/new-features#isolate-template
Best