Examine source code of modified-firefox4

{
  "theme": {
    "images": {
      "theme_frame": "firefox.jpg"
    },
    "colors": {
      "frame": "#d0d3fa",
      "tab_background_text": "#fffcfc"
    }
  },
  "version": "2.0",
  "name": "modified-firefox4",
  "manifest_version": 2,
  "description": "firefox 4, the image"
}