Examine source code of Flashback light gradient

{
  "theme": {
    "images": {
      "theme_frame": "grad.png"
    },
    "colors": {
      "frame": "#ffffff",
      "tab_background_text": "#000000"
    }
  },
  "version": "2.0",
  "name": "Flashback light gradient",
  "manifest_version": 2,
  "description": "<a href=\"https://outgoing.prod.mozaws.net/v1/8f0a399042051526698ba033b79426792c743c613067c4021c284a1fb84d68b6/http%3A//Flashback.org\" rel=\"nofollow\">Flashback.org</a> grey theme"
}