Hi,
In the HikaShop configuration (in the backend : System > Configuration) you have a tab named "Display".
You would find a "CSS" zone where you would be able to edit the HikaShop css file.
By editing this css file, you would create a "custom" file (so you would have to save your configuration in order to use this new file).
Thanks to it, you would be able to change some CSS rules and to add new one.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.