Examine source code of Responsive Grid

{
  "theme": {
    "images": {
      "theme_frame": "FF-responsive-persona-header.png"
    },
    "colors": {
      "frame": "#c2c2c2",
      "tab_background_text": "#ffffff"
    }
  },
  "version": "2.0",
  "name": "Responsive Grid",
  "manifest_version": 2,
  "description": "This persona gives you a set of grid lines indicating approximate browser viewport widths which can be very useful when designing responsive websites. Works especially well in combination with the Web Developer toolbar for CSS, so you can quickly drag the sidebar break to whatever width you want to work in: longer explanation of how I use it here: <a href=\"https://outgoing.prod.mozaws.net/v1/ad472af08c1457d744988468051099409de0dcaa82cce8a7f131a81560497025/http%3A//www.jamessmith.co.uk/articles/responsive-firefox-persona\" rel=\"nofollow\">http://www.jamessmith.co.uk/articles/responsive-firefox-persona</a>"
}