Examine source code of Theme Switcher

{
  "manifest_version": 2,
  "name": "Theme Switcher",
  "description": "Simple utility to switch between light and dark themes",
  "version": "0.0.3",
  "icons": {
    "48": "icon_light.png",
    "96": "icon_light.png",
    "192": "icon_light.png"
  },
  "browser_action": {
    "default_icon": "icon_dark.png",
    "default_popup": "popup.html"
  },
  "permissions": [
    "*://duckduckgo.com/*",
    "cookies",
    "theme"
  ]
}