Examine source code of Developer

{
  "theme": {
    "images": {
      "theme_frame": "g-gecko-dev-header.jpg"
    },
    "colors": {
      "frame": "#333333",
      "tab_background_text": "#ffffff"
    }
  },
  "version": "2.0",
  "name": "Developer",
  "manifest_version": 2,
  "description": "A dark gradient header and light gradient footer image for sharp contrast. Useful for viewing Firefox add-on's that sometimes get lost in dark footer bars."
}