A Zendesk theme is what is used in Zendesk Guide to build the support portal for the Zendesk instance.
Currently, we use a custom Zendesk theme for our support portal. The theme is built using the Mink theme as a base. From there, we have customized it to specifications from both the UI/UX team and the Support team.
You can locate our current theme via the zendesk-theme project.
NOTE: Currently, all changes made in the above project have to be manually applied in the Zendesk instance itself.