You can put override K2 css into
ROOT/templates/ja_purity_ii/css/template.css file
no need put into k2.css
Exp: my template.css start line 800
div.catItemHeader h3.catItemTitle {
font-size: 24px;
}
more you can see my site :
www.chooto.com.vn - using firebug to edit css from front end