I was helping @Micha to upgrade his old wBB2 to XenForo and with that, we were fiddling around with some templates. While I know from betas and RCs that there should be a button to add new template mods, there is none in 1.2.0 in none of the forums I have access to. It looks like on the picture.
If you want to use TMs to edit your templates as an end user, we're not going to outright stop you - hence the direct URL access. We are going to discourage it though as the template should be edited directly instead.
That's not the first time I see people wondering where is 'Create new modification' button. Maybe it's better to add some faint explanation there like: 'You need to enter debug mode in order to create new TM'.
Is there any particular reason why users should not be creating template modifications and editing the templates instead? I use the Template Modifications to test things, and it is very easy being able to turn something off with a click of a button.