We’ve just uploaded a new Carto stylesheet to GitHub - Open Streets. The style is similar to the Open Streets DC style that comes with TileMill, but it is designed to work with an OpenStreetMap PostGIS database rather than with shapefiles. If you have a database created by the osm2pgsql import tool, with some quick configuration to match your setup you can be up and running with this style in TileMill in a few minutes.
Here is a small demo of the style showing an area I frequently edit on OpenStreetMap, my home town of Barrie, Ontario. The style is a work in progress, so certain things are still missing. Keep an eye out for updates as we continue to add features and refine the design.