Thanks,
As already indicated, it's your template.css applying different background colours to different types of <input> tags.
For the widths, the (Joomla) form fields for the first part (username, password, etc) have size attributes set, while the (HikaShop) address form fields do not have this.
All of this can be corrected and standardised with custom CSS. I've given you already a link to the relevant part of the documentation here on the site, and since you're using Firefox anyway, I encourage you to install and use Firebug to even better see and analyse present CSS, as well as simulate CSS changes and amendments.