Examine source code of Add-ons for Firefox

{
  "theme": {
    "images": {
      "theme_frame": "header.png"
    },
    "colors": {
      "frame": "#eb5299",
      "tab_background_text": "#0a0200"
    }
  },
  "version": "2.0",
  "name": "Add-ons for Firefox",
  "manifest_version": 2,
  "description": "Create a New Theme\n\nBackground themes let you easily personalize the look of your Firefox. Submit your own design below, or learn how to create one!"
}