Hi,
If you override the CSS file using the HikaShop backend interface, your css file should be kept.
If you modified directly the main CSS file, it is automatically overwritten during updates.
HikaShop 2.3.0 include a lot of design improvement but we let the old CSS file in order to let users have the exact same file than before.
So you can use the CSS "old" in your HikaShop configuration in order to use the same CSS file than it was in HikaShop 2.2.x
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.