I swear this should be simple, but something isn't clicking. I've followed your instructions, with matching code names (I'd tried this before just guessing actually)...but I don't see the "Theme" tab anywhere except on Page Templates or Page Layouts.
Stylesheet - https://db.tt/dyZYosx5
Site - https://db.tt/Pso2rkjl
An additional note, there will be 3 sites on the platform, hopefully all sharing the same parent theme to some degree. The theme also needs to support more than a single stylesheet, currently using a collection of LESS files that rollup into a single reference, I'm not sure how that plays into this.
I'm using Visual Studio 2013 so the CSS stylesheets do not have to live 100% in the database and I would prefer to edit them in Visual Studio.
Appreciate the help for a Kentico newbie.