Category with product list, columns not responsive

-- HikaShop version -- : 2.5.0 - Starter
-- Joomla version -- : 3.4.3
-- Browser(s) name and version -- : Chrome 43.0.2357.132

Hello,
I've just setup a menu item to display a category with product listing (thumbnail + title) in 3 columns.
The issue is that the columns seems not to be responsive, they just shrink when you reduce the screen size.

See here: infinitemandala.com/shop/opere-arte/cate...-i-colori-della-luce

I inspected the code (even if I am not a programmer, but I know a bit of css) and these columns are infact not fluid, there is a "width:32%" style for each column, I tried to override it with a "width:100%" with mediaqueries for tablet and phone screen sizes, but with no luck since now.

Are this columns not supposed to be responsive or am I missing something here?

Thank you

Nic

SOLVED

I needed to check "use Bootstrap design" in
System > Configuration > Display > General Display Options