This is from the "what's new" document from v1.3.5 concerning stylesheets:
CSS-per-page coding changes may affect your stylesheet naming conventions. The naming for category pages now properly uses cPath, like: c_3_10.css, c_31_52_88.css, etc.
I'm guessing that it means you need to specify the complete $cpath for each category/subcategory combination.

For your hardware page

c_1.css would be for your top category.

c_1_17.css would be the first subcategory in that top category.

I'll play around with this a while and see if I can work out something.

In the mean time maybe DrByte can come up with something.