How can I override with css the class .hikashop_product_image?

-- HikaShop version -- : HikaShop Business: 4.0.3
-- Joomla version -- : Joomla 3.9.6.
-- PHP version -- : 7.2.13

Hello Nicolas, maybe the solution is very easy but I am having a bit trouble with the web responsive, when the device view becomes smaller the white space under the product image is more because its container has a fixed height. How you can see in the first photo there are the white space under the product image because the container with class .hikashop_product_image has got height with a fixed value. In the second photo the white space there is no more because through the inspect element of Chrome I modified the class by removing the fixed height and now It is responsive. I tried to do the override on my user.css file but doesn't work. The third photo is from Chrome after my override, the second row on the image is my override but it doesn't override the first row (.hikashop_product_image). If I Don't remove that fix height my responsive web doesn't work and the view is very bad. I understood that the css class .hikashop_product_image is in the file index.php.There is something that does not allow me to overriwrite the file. But how can I overwite it if it is possiblle? This is the link of my product list www.tozainetwork.net/brainstorming/index...g&Itemid=469&lang=it
I hope my request is clear. Thank you for your great support.

Thank you Philip. Now It works. I had to write the statement important!. Kind Regards.