bad display of hikashop registration form

-- HikaShop version -- : 3.1.1
-- Joomla version -- : 3.7.2
-- Browser(s) name and version -- : all

Hello,
Can you tell me were is the code that manage the Hikashop registration form.
With the template of the site the display is very bad (see screeshot at prntscr.com/fn76cd )
I plan to rewite a custum css code for site template
Thanks
Alain