Examine source code of Sepia Adjuster

{
  "manifest_version": 2,
  "name": "Sepia Adjuster",
  "description": "Firefox is really powerful, and this extension pushes it even further!!! The Sepia Adjuster tool, helps to change sepia level in websites. Just open any website and open the sepia adjuster extension. There, adjust the slider to change the sepia value, it's that simple!!!",
  "version": "1.0.0",
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "/content_script.js"
      ]
    }
  ],
  "browser_action": {
    "browser_style": true,
    "default_icon": {
      "354": "/icon.png"
    },
    "default_title": "Sepia Adjuster",
    "default_popup": "/popup.html"
  },
  "permissions": [
    "storage"
  ]
}