doqment is a modified version of Firefox's own PDF viewer, with a Reader Mode add-on that allows you to view PDFs in different colour themes. It has both Light & Dark themes to make long-time reading more comfortable on your eyes. It also adds some other handy features like Smart zooming text and hiding the toolbar to assist reading, especially on smaller screens. Happy PDF Reading! Keyboard shortcuts (Gestures):

Alt+Q: open a new PDF reader tab (^Q on macOS).
Z: smart zoom text under mouse pointer (double tap).
F3: show/hide the main toolbar (scroll).
Ctrl +/-: zoom in/out the PDF (pinch).

All other Firefox PDF viewer shortcuts should work the same in doqment too. Viewer preferences can be changed by visiting Manage Extension => Preferences (on desktop); or Extensions Manager => doqment PDF Reader => Settings (on Android). Notes:

It is recommended to disable the built-in viewer for downloaded PDFs while this extension is enabled. Go to 'Settings', scroll down to 'Applications', and set the 'Action' option for 'Portable Document Format (PDF)' to something other than 'Open in Firefox' (alternatively, power users can toggle pdfjs.disable in about:config).
Changing the colours of PDFs needs some processing and that may slightly slow down the rendering in Reader Mode. Feel free to report if it is significantly slow for a PDF you are trying to read.
Please do not report your issues with this extension to the Firefox viewer team. This is a separate extension, so post your issues/feedbacks directly at AMO, by mail or over at the GitHub page.

This software is fully open-source and does not collect any user data.