Hi,
That's not a bug actually.
Since you didn't enter yet any address for your account, it's asking you for one. In order to make it more easy for you to fill it, it takes the name that you entered in your joomla user account, split it, and use the first part as the lastname and the second part as the firstname in order to prefill the fields.
Since you didn't specify both, it just fill the lastname field with your joomla user account name.
That's not really a problem since you can simply change the value there to what you want. Also, new users will have to enter all the information when they register during the checkout and users who already did that will already have their address filled out.
For the text styling in the popup, it uses the styling of your template :
www.breakoutbox.co.uk/templates/atomic/css/template.css
So you need to add your text CSS in there.