Please check:
Selecting the correct template on the backend (K2 → Categories → Edit category → Variety of category elements (tab on the right) → Select template).
If you do not see the name of your custom template, double-check the path to the template.
To have another css file (one for each category template, if you want) from K2, install this plugin:
http://www.jiliko.net/extensions/css4k2.html
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