I created a new k2 template and I assigned the category and the menu to the new template. I set the tags for each article in the category and I modified the category_item.php file in the folder /templates/mytemplate/html/com_k2/default/
But the css class is always "catItemView".
this is the link of the site is still under construction: alternativo.fablabsassari.org

Try to add some classes (dummy ones) to see if they show up.

tried to add css classes with the same name of the tag in my custom.css and k2.css. nothing has changed :(

Sorry, I meant to add a dummy class to the category item so I can see that the correct file is being edited.

I added the class "test" with white color background. Could you check? Did you mean this?