I'm working with Joomla 1.5 and K2 2.5 and I don't know how to give an specific style to a category, I still don't understand the "override" thing.
I've been following this tutorial:
My categories still have all the same style, what I'm missing there ?
I've been carefully (I think so) placing all files with no success so far.
Please check:
Choosing the right template on the backend (K2 -> Categories -> Edit Category -> Category Item Layout (Tab on the right) -> Select a template).
If you don't see the name of your custom template there, then double check the path of your template.
In order to have a different css file (one for each category template if you want) from the K2 one, install this plugin: