Hello,
You don't have to change the default Css command (from HikaShop), you just have to add your custom Css at the end of your css file & with a good selector your custom command will override previous.
To define a powerful selector, you can use your navigator inspector tool (point html element on your website page and right-clic), like on this screenshot to find the good html class :
I advice in order to increase selector priority, add the concerned class page, as by example something like this :
.html_page_class .html_price_class {
font-size: 1.3em;
font-weight: bold;
color: red;
}
Hope this will help you.
Regards