Hi,
You cannot modify or deactivate "hikashop.css" file ; it is a core file which provides essential elements to allow HikaShop to be displayed correctly.
I would recommend you to use a dedicated plugin to handle the CSS files and provide a merge & minified version.
But because the "hikashop.css" file can evolve and the modification would be required to allow HikaShop to work properly, that version is not allowed to be removed so we are sure that the modifications will be always included. So even if you have a CSS customized, you will be up to date for the core CSS rules.
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.