How do I repair permissions in MacOS High Sierra?

The installer repairs system permissions (if they ever needed to be fixed). It is quite difficult to alter the system permissions, so there isn't any functions to do that.

You can reset your home folder permissions: Resolve issues caused by changing the permissions of items in your home folder - Apple Support

Repair Home Folder Permissions

One no longer can repair permissions in Sierra or High Sierra. The system files are locked by the SIP to prevent the permissions from being altered even by someone with admin control.