Examine source code of Pixels Rule

{
  "theme": {
    "images": {
      "theme_frame": "header.png"
    },
    "colors": {
      "frame": "rgba(229,230,232,1)",
      "tab_background_text": "#000"
    }
  },
  "version": "2.0",
  "name": "Pixels Rule",
  "manifest_version": 2,
  "description": "Check your window width with this handy pixel ruler."
}