Sites custom codes - inject CSS and JS

manifest.json

{
  "update_url": "https://clients2.google.com/service/update2/crx",
  "name": "__MSG_extension_name__",
  "description": "__MSG_extension_description__",
  "author": "Somonator",
  "version": "0.4",
  "default_locale": "en",
  "manifest_version": 2,
  "minimum_chrome_version": "55.0",
  "icons": {
    "16": "icon.png",
    "48": "icon.png",
    "128": "icon.png"
  },
  "permissions": [
    "tabs",
    "storage",
    "unlimitedStorage",
    "<all_urls>"
  ],
  "options_page": "options-page.html",
  "browser_action": {
    "default_icon": {
      "19": "icon.png",
      "38": "icon.png"
    },
    "default_popup": "popup.html"
  },
  "background": {
    "scripts": [
      "extension/api.js",
      "extension/background.js"
    ],
    "persistent": true
  },
  "content_scripts": [
    {
      "run_at": "document_start",
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "extension/inject-scripts.js",
        "extension/content.js"
      ]
    }
  ]
}