Make every website properly dark, not just dimmer
Half the dark mode extensions just turn the brightness down. The screen is still lit. It just looks tired.
Key Insights
Dark Reader and Stylus both invert or restyle pages, with results that vary by site. Pitch Please aims for one thing, true black, and does it in one click.
The short version
All four darken bright pages, but they reach that goal from different directions. Pitch Please forces any page to genuine black in one click, which matters on an OLED screen where a black pixel is actually switched off. It works instantly with no per site setup. Dark Reader inverts and adjusts colours automatically, which is convenient and occasionally guesses wrong on a chart or a logo. Stylus applies user stylesheets you find or write yourself, which gives full control at the cost of needing CSS. Night Eye sits between the two, offering automatic dark themes with some manual tuning available.
What actually separates them
The difference is what gets changed and how much control it asks for. Pitch Please rewrites the page itself to black on demand, with one click and no configuration. Dark Reader and Night Eye both automate the same idea with per site adjustment, which is more flexible and also more likely to misread an image or a coloured graphic. Stylus asks you to supply that judgement yourself in CSS, which is powerful for someone willing to write it and a wall for anyone who is not.
What you get free
Pitch Please is $0, free with no account, and asks for only 2 permissions on install, storage and alarms. Dark Reader and Stylus are both free outright. Night Eye offers a free tier with its paid plan unlocking extra themes and scheduling options. None of the four meter how often you use them once installed.
Strengths and weaknesses
Verdict

Dark Mode, Everywhere

Pitch Please wins for anyone who wants genuine black on any page with nothing to configure first.
Who should pick what
Someone reading at night on an OLED screen should install Pitch Please and stop configuring things. A person who wants automatic per site tuning and does not mind the occasional inverted image should use Dark Reader. Anyone comfortable writing CSS for sites they use daily will get more from Stylus. Night Eye suits someone who wants automatic dark mode with the option to fine tune it later.
Absolute beast