properties.schema

{
  "type":"object",
  "$schema": "http://json-schema.org/draft-04/schema",
  "id": "http://jsonschema.net",
  "$ref": "http://localhost/plugins/content/theme/model.schema",
  "properties":{
    "_screenSize": {
      "type":"object",
      "required":true,
      "properties":{
        "large": {
          "type":"string",
          "required":true
        },
        "medium": {
          "type":"string",
          "required":true
        },
        "small": {
          "type":"string",
          "required":true
        }
      }
    }
  }
}