Hi,
1. You can't set a border around the boxes of items.
2. The size of the thumbnails can be changed in the config under the tab files. You will have to delete the folder media/com_hikashop/upload/thumbnails so that hikashop can automatically recreate the thumbnails with the new size.
3. You need to go in the menu Display->Content modules and edit the module with the id 80. There, you will be able to personalize the display of your products (number of columns, margin between products, background, rounded corners, etc.
4. The login screen on your inloggen page is a joomla login page. You will have to edit the joomla login view if you want to change it but that question should be better answered on the joomla forums