go to file system and use a tool to find a file with *.css extension and that contain on of this string hikashop_products or hikashop_product.
this string must already exist since it must be define elsewhere .
Because important ! is use it mean this code already exist somewhere ( in frontend_default.css ? )
in this file code paste the selector definition where ever you want (important to take care of order ) .
make sure the css file you find by this way is load in the client browser when you load the list of product .
refer to the hikashop demo page the css file name can be locate in media/com_hikashop\css\frontend_default.css or frontend_custom.css
CSS recommandation style guide is to put non hikashop code to another css file of name front_end_custom.css but be shure that Hikashop is set to load this file .
The link (CSS) give by Mohamed thelji show a capture screen that show that default css is load ( frontend_defautl.css)
Component -> hikashop -> Configuration
then display ( tab) -> CSS ( field border) -> FrontEnd File set to default ( list )
the link " Easily adapt HikaShop to your template with Styles" ll not help you .
The best way to learn is to do it once by yourself and then buy any template since you know howto write a template .
understand template need understand html stream , css and joomla cms .
regard's