On the shop page of the e-commerce module, I would like to change the class of

<div id="products_grid_before" class="col-md-3 col-sm-4 hidden-xs">

I would like to change `hidden-xs` to `show-xs`. I created the following xpath selector:

<xpath expr="//div[@id='products_grid_before']" position="attributes">            
   <attribute name="class">col-md-3 col-sm-4 show-xs</attribute>         
 </xpath>

However the class doesn't change. The xpath selector is placed well in the code and works as expected if I change 'name' to, say 'a'.

How can I change the class of #products_grid_before?

Thank you!