Hi,
See :
www.hikashop.com/blog/367-hikashop-3-0-out.html#legacy
To use the new settings for the "CSS for buttons", you need to use the new "add to cart" system of HikaShop 3, so you need to not have the legacy settings.
Then, it will be possible.
Regards,
PS : I'm just paraphrasing Nicolas' message.
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.