I didn’t at the time of writing (as I thought Justin meant to just include that function) however, although including the whole file got it to work - it wasn’t the same as when I directly edited the file.
For instance it worked on the latest page, but not in the all categories drop down… which did work when I edited the file directly!
So I’ve ended up making the changes I needed via some messy CSS 
and I’ve had to make some compromises too - it’s definitely better than not being able to have done it, but ideally I would have preferred something as simple as changing a category style to be as simple as editing a template 