Examine source code of Books

{
  "name": "Books",
  "manifest_version": 2,
  "version": "1.0",
  "theme": {
    "images": {
      "theme_frame": "Firefox-Theme---Books.jpg"
    },
    "colors": {
      "frame": "rgba(211, 219, 235, 1)",
      "tab_background_text": "rgba(124, 150, 237, 1)",
      "toolbar": "rgba(150, 57, 57, 0.25)",
      "toolbar_field": "rgba(168, 185, 255, 1)",
      "toolbar_field_text": "rgba(255, 10, 39, 1)"
    }
  }
}