Hi,
1. Edit the module via the Joomla modules manager, and under the "hikashop options" tab, you can turn off the display of the custom item fields. It's also possible to hide custom product fields with CSS specific to the module.
www.hikashop.com/support/documentation/1...ize-the-display.html
2. You can add your CSS either in HikaShop or in the template, both will work as long as the CSS file is loaded is on the pages where you want your CSS to apply.
The link above explains how to add your CSS to HikaShop.