Examine source code of DesignYourTube

{
  "name": "CustomTube",
  "version": "1.0.4.4",
  "description": "Choose your own YouTube layout, and more!",
  "permissions": [
    "activeTab",
    "storage"
  ],
  "browser_action": {
    "default_popup": "popup.html",
    "default_icon": {
      "16": "images/16.png",
      "32": "images/32.png",
      "48": "images/48.png",
      "128": "images/128.png"
    }
  },
  "icons": {
    "16": "images/16.png",
    "32": "images/32.png",
    "48": "images/48.png",
    "128": "images/128.png"
  },
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "content_scripts": [
    {
      "matches": [
        "*://*.youtube.com/*"
      ],
      "exclude_matches": [
        "*://*.youtube.com/embed/*"
      ],
      "css": [
        "general.css",
        "css/bt-universalized-elements.css",
        "css/static.css",
        "css/static-scrolling.css",
        "css/psuedo-static.css",
        "css/rich-grid.css",
        "css/topbar.css",
        "css/sidebar.css",
        "css/chips.css",
        "css/search.css",
        "css/watch-page.css",
        "css/watch-page-blobs.css",
        "css/ltod.css",
        "css/description.css",
        "css/comments.css",
        "css/playlist-panel.css",
        "css/channel-rework.css",
        "css/related-videos.css",
        "css/cosmicpanda.css",
        "css/polymer.css",
        "css/sub-btn.css",
        "css/menus.css"
      ],
      "run_at": "document_start"
    },
    {
      "matches": [
        "*://*.youtube.com/*"
      ],
      "exclude_matches": [
        "*://*.youtube.com/embed/*"
      ],
      "js": [
        "initial-setup.js"
      ],
      "run_at": "document_start"
    },
    {
      "matches": [
        "*://*.youtube.com/*"
      ],
      "exclude_matches": [
        "*://*.youtube.com/embed/*"
      ],
      "js": [
        "main.js"
      ]
    }
  ],
  "web_accessible_resources": [
    "images/*"
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{7917c4c9-c063-427a-9d29-15dc997789ff}"
    }
  },
  "manifest_version": 2
}