The default module.css file is located in: mods/com.phenixidentity~phenix-prism-enroll-oath~<version>/web/css. This file should not be changed since it gets overwritten during an update of the server. The file, however, can be copied and placed in the overlay location: overlay/activatepocketpass/css
When doing so, the overlay file will "replace" the original file.
The recommended way to change/override css is to find the customer.css file in: overlay/activatepocketpass/css folder and add the customisation there. This file is loaded after the original file and only the configured css values will override the default css values.
All changes in css files will require a restart of the service
How to use
Best practice is to copy the css element from the original css to the customer.css in the overlay/activatepocketpass/css folder and start from there
Regarding images and other references in css
Customised images (and other files) can be referenced in the css with the base path: prism/overlay/images/
which points to the overlay/activatepocketpass/images folder.