Hi,
I think you're talking about HikaShop 2.3.4 and not 2.2.4.
If you used the CSS override system in HikaShop ; you won't have any problem.
The update will update all HikaShop core files but all of your overrides (views, css) and configuration will be kept.
About "bootstrap.min.css", it is not related to HikaShop but to Joomla (or your template, it depends).
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.