It is not possible to upload a previously built template/style into the Sitebuilder.
Our Responsive templates, in conjunction with our Style Designer, are our most flexible templates yet. You are able to control almost every element on your site. These templates are able to adapt to any screen size resolution! These templates can be found by going to Style > Change Template and selecting the Responsive folder.
If you go to Style > Designer, you will see a wealth of options available for you to tweak your site.
In addition, depending on the type of package you have purchased, you may have access to the Site CSS feature which allows you to customize your site even further.