Examine source code of Things are grey

{
  "theme": {
    "images": {
      "theme_frame": "grey_header.jpg"
    },
    "colors": {
      "frame": "#262222",
      "tab_background_text": "#ededed"
    }
  },
  "version": "2.0",
  "name": "Things are grey",
  "manifest_version": 2,
  "description": "Quite minimal :]"
}