gistfile1.txt

ns29899(mongod-2.4.8) lichess> db.pref.aggregate({$group:{_id:'$theme',nb:{$sum:1}}})
{
  "result": [
    {
      "_id": "canvas",
      "nb": 1163
    },
    {
      "_id": "green",
      "nb": 1333
    },
    {
      "_id": "blue",
      "nb": 1167
    },
    {
      "_id": "grey",
      "nb": 648
    },
    {
      "_id": "brown",
      "nb": 18032
    },
    {
      "_id": "wood",
      "nb": 2113
    },
    {
      "_id": null,
      "nb": 22434
    }
  ],
  "ok": 1
}