Hi,
When the user enter his username and his password ; HikaShop call Joomla for the authentication.
So it's Joomla which said that the username and password do not match, etc.
For the popup and the colors ; it's related to your template, not to HikaShop.
And if you want a "guest checkout" ; it's possible but it requires HikaShop Business.
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.