You mean to rename k2.css file (same name as template) in html/css folder? Then pick it with this plugin?
Because if you don´t do this K2 pull that css for whole extension.

I have created a folder in modules/mod_k2_content/tmpl called monthly focus and copied the k2.css file into this directory.  When I open the module I see the monthly focus in the dropdown of sub-template types.  I choose it but can not see where my changes are occurring.  I have changed the background color on the even and odd for the "Zebra Rows".  Can anyone provide any help.