If you allow me to do a bit of self-promotion: https://code.mschae23.de/mschae23/mlmym (this is my fork of the software running on old.lemmy.world).
It's a different frontend rather than just a theme, so it needs to be set up separately (similar to lemmy-ui). If you have any questions, feel free to ask me :)
Yeah. I believe it has a different name on iOS, but they both have the feature.
And I think iOS literally lets you configure the button on the side of the phone to enable this mode by quickly pressing it three times or something like that.