Custom Files is located under My Calendar > Help > Custom Files.

The Custom Files screen provides guidance on where to place custom style sheets, icons, and templates. If you want to manage custom styles, use this information to determine where to store your custom stylesheet. If you want to replace the default icon collection with your own custom icons, use that file path.

For PHP templating, this tells you where to store them; but you’ll get more information in the PHP templating documentation.

A view of the information found at My Calendar > Help > Custom Files, showing locations to place custom files for use by My Calendar.

Next: Go to Help > Privacy.