Hi,
That doesn't work because of the priority of the CSS code. Since the code of the template already has a !important, then the priority goes to the other CSS code which is directly in the header instead of the CSS in the custom.css file.
So instead, to get the higher priority, you need to target more specifically the price.
For example:
.module_round_box-highlight1 .hikashop_product_price {
color: black !important;
}
should help.
I would recommend to check with your template provider otherwise. They'll know better than us how to properly override the CSS on their template.