For some reason I see data-template-name attribute on many html elements. I don't see it here in xenforo source code. I have debug, dev and designer disabled. @Russ does this have to do with xenbase?
Go to the admin CP, Options, Appearance...you'll find the following option. Alternatively, you can use the admin panel search function to search for "HTML", and find the option that way.
If you don't see it, initially, you may need to enable advanced options by clicking this link below the save button.