I believe most themes and color schemes work fine with ST2. Structurally, ST3 is the same as ST2. Packages will go into the /Packages folder. You can place your user specific stuff in /Packages/User. I'm guessing you used Package Control in ST2, so why not install it for ST3 as well? It is more of a manual installation process right now though. Before installing a package using package control, you may need to add the ST3 branch of a repository. Don't be alarmed by the lack of packages in the /Packages folder. In ST3, packages are run directly from the .sublime-package files, rather than needing to be extracted.