Oddbean new post about login | logout @ploum I know you've mentioned in blog posts using a black and white phone, thought I would ping you in case you found this interesting or useful. Probably would be possible outside of Graohene as well.
There are some limits since app developers need to support these themed icons in their apps sadly. If you want total grayscale on everything, there is a Grayscale mode in color correction accessibility settings which you can also shortcut to turn on and off. Note taking screenshots, pictures etc would still be in color, though...