Examine source code of NekaBorder

{
  "manifest_version": 2,
  "name": "NekaBorder",
  "version": "3.17.3",
  "homepage_url": "https://sites.google.com/view/nekaapps/home?authuser=1",
  "description": "Custom borders on sites",
  "icons": {
    "48": "images/border-48.png"
  },
  "content_scripts": [
    {
      "matches": [
        "*://*.google.com/*"
      ],
      "js": [
        "border.js"
      ]
    },
    {
      "matches": [
        "*://*.mozilla.org/*"
      ],
      "js": [
        "borderify.js"
      ]
    },
    {
      "matches": [
        "*://*.weebly.com/*"
      ],
      "js": [
        "borderify.js"
      ]
    },
    {
      "matches": [
        "*://*.ixl.com/*"
      ],
      "js": [
        "borderify.js"
      ]
    },
    {
      "matches": [
        "*://*.wikipedia.org/*"
      ],
      "js": [
        "borderify.js"
      ]
    },
    {
      "matches": [
        "*://*.freerobuxcodes.net/*"
      ],
      "js": [
        "badsite.js"
      ]
    }
  ]
}