The priority was set to a low value while it was meant to be a high
value. Indeed, we want that if that template is active, the customize
modal is emptied no matter what, so it has to be called last.
This is to have an understandable behavior if our code fails to mark the
template as inactive on a theme installation. With this commit, the
customization dialog will be empty. Without this commit, it gives an
error which can lead to think we made a mistake writing the theme's own
customization dialog while it was in fact inheriting from an emptied
dialog.