CSS file that applies to all websites on the Internet

 Yes. You can make an Edge Add-On/Chrome Extension. Chrome extensions are compatible with Edge.

You will need this in the manifest.json file:

{
  "manifest_version": 3
  ...
  "content_scripts": [ 
    { 
      "css": ["yourCSS.css"], 
      "run_at": "document_idle", 
      "all_frames": true, 
      "matches": [ "https://*/*" ] 
    } 
  ]
}

You will then need a file in that directory called `yourCSS.css` or whatever you want to name it, with whatever CSS you want. You might need to adjust "run_at" and "matches" for it to work properly based on your use-case.

{
  "manifest_version": 3,
  ...,
  "permissions": ["scripting"],
  ...
}

Heinz_Berecz