Splitted images on user cpanel header

That's a CSS problem between the CSS of HikaShop and you're template's. I would need to be able to see that page with firebug in order to tell you what CSS to modify.

The border of the header can also be modified with CSS.

For example, on the addresses listing of the user it's:

#hikashop_address_listing fieldset{ border: 1px solid black; }

For the cpanel, it's #hikashop_cpanel_title fieldset{ border: 1px solid black; }

That CSS doesn't exist. By changing it and adding it to the CSS in your template or in HikaShop (via the interface in the display tab of the config), you can change the display of the border of the fieldset.