https://addons-legacy.palemoon.org/addon/pdf-js-for-seamonkey/
Addons like Dark Background And Light Text used to be able to also works on PDFs. The Swarth fork of Dark Background And Light Text doesn't work on PDFs anymore.
https://addons-legacy.palemoon.org/addon/swarth/
On Firefox, there is this trick to do so:
https://pncnmnp.github.io/blogs/firefox-dark-mode.html
Does this trick work with Pale Moon PDF reader addon, too?
I think it's better to consult the Pale Moon experts first. They banned me on their forum so I can only ask here.
I confirmed this trick works on LibreWolf.
Due to server slowness, downtime, and other issues, guests can no longer search the forum or view user accounts. At some point in the future Eclipse will be moving to a more stable and efficient platform that should result in much better stability and the lifting of these restrictions. There is no timeline for this yet, just want you to know what's happening with all the downtime and I have a plan to fix it.