Dark Mode for PDFs on Firefox

firefox-pdf-dark-mode.txt

From: https://stackoverflow.com/a/74557483
Copying to keep track.

This is how to make the full page dark

Tutorial
Add a new Bookmark
Copy the following snippet: 
javascript:(function(){document.body.parentElement.style = 'filter: grayscale(1) invert(1)';})()
Paste it into the bookmark's address
Save the bookmark
Whenever you're on a PDF, click on the bookmark
Explanation
This is a bookmark that executes Javascript (this is why it starts with javascript and not with http or https). In the Javascript code we get the root html tag (document.body.parentElement) and change it's style .style = 'filter: grayscale(1) invert(1) contrast(0.8)';.

That's literally everything :)

Edit:
Removed "contrast(0.8)" from code

To revert:
Add another bookmark that has the following URL:
javascript:(function(){document.body.parentElement.style = 'filter: grayscale(0) invert(0)';})()