Examine source code of Outlook

{
  "theme": {
    "images": {
      "theme_frame": "header.png"
    },
    "colors": {
      "frame": "#222222",
      "tab_background_text": "#ffffff"
    }
  },
  "version": "2.0",
  "name": "Outlook",
  "manifest_version": 2,
  "description": "A simple theme based on a popular skin available in Microsoft <a rel=\"nofollow\" href=\"https://outgoing.prod.mozaws.net/v1/5df07da2d9f137ab21d4a4d63cb97500a3633181a2736883da16dbe5aeb56149/http%3A//Outlook.com\">Outlook.com</a> or Outlook Web App"
}