Marxists.org is notoriously an eyesore. Someone on GitHub by the name of ArkansasWorker made a custom CSS for the website that makes it more readable. I forked their project for some minor changes based on personal preferences (for example I prefer serif font on light mode and I remember there being problems customising the font in the settings).
Installation
First you will need to install Stylus for Firefox. There must be a Chrome counterpart extension as well but I am not sure.
After that click on ONLY ONE of the links below based on whether you the original or my fork:
This should open the file in Stylus automatically and you can install with the install button on the left hand side.
You can also customise the layout using an intuitive menu. The instructions for that are described here: https://github.com/ArkansasWorker/marxists.org-UserCSS#customizing-user-styles
You can do this if you, for example, want to make the text column wider without having to edit the CSS by hand.
Screenshots
Original
Custom CSS
Swiss theme
Dracula theme
Firefox carrying the good browsing experience since Netscape’s demise. Reader mode is also really good on mobile.
I just wish it had the article TTS like it does on desktop!