You can also use Mapbox Studio Classic in combination with OpenMapTiles to create Tilemill2 tile styles for Mapnik renderer.
NOTE: If you need raster tiles it is highly recomended to use the TileServerGL project, which provides raster tiles rendered on server side from the vector tiles and the modern GL styles.
Change the Source
Go to Vector tile hosting and get a free key. Then download tilejson from URL
and save it as a file with
.json extension (the extension is necessary for Mapbox Studio Classic).
Upload the JSON to the web, e.g. using Gist.
Finally change the source of your TM2 style to URL of your tilejson and save the style.
Check out the video tutorial how to port an existing TM2 style to OpenMapTiles.
Ported Classic Styles
We ported over some nice looking open licensed raster tile styles.