I have enabled the dark mode in Firefox version 76.0 as well as installed the dark reader extension. The extension allows web pages to go dark. These changes do not affect any PDFs that have been opened in Firefox.
Create a bookmarklet - a bookmark with the following in the location/address/URL field:
javascript:(function(){var el = typeof viewer !== 'undefined' ? viewer : document.body; el.style.filter = 'grayscale(1) invert(1) sepia(1) contrast(75%)';})()
Then, click on this bookmark after you open a PDF in Firefox.