Theme setting for card background

The July 2018 release provides support for background colors.  The snippet below has a dark grey background with a lighter grey area surrounding it.  Enjoy.

    "*": {
      "*": {
        "*": [{
            "fontSize": 8,
            "fontFamily": "Arial",
            "titleFontFamily": "Arial",
            "color": {
              "solid": {
                "color": "#77777"
              }
            }
          }
        ]
      }
    },

    "page": {
      "*": {
        "background": [{
            "color": {
              "solid": {
                "color": "#333333"
              }
            },
            "transparency": 0
          }
        ],
        "outspace": [{
            "color": {
              "solid": {
                "color": "#444444"
              }
            },
            "transparency": 0
          }
        ]
      }
    },