What is the best theme to work with multisite with user generated stores like etsy

I am working on a new project and am really new to this stuff. I have set up my mutlisite and all the options are ready. Now i need to build!

The first thing i want to tackle is the sign up page for both storefronts and users. I need this so i can get my first pilot customer to use the site for testing.

I have been researching for a few days and have found myself going in circles. I want to create an "Register" site where users login and create their account (which can be easily done with a one click with their facebook, or they can sign up the regular way) and then also a sign in with the same option. Like the screen shot at the bottom.

What is the most customizeable theme to use for this?

Most of the searches i have done on this site for help turn up posts from 2011 and 2012, you might want to archive those since your products have changed. It makes it really confusing.