https://twitter.com/ParkerNathans/status/1435270801905831942
*##html:style(filter:grayscale(1))
localhost##html:style(filter:grayscale(0))
... etc.
st = !window.st;
document.querySelector("html").style.setProperty("filter", st ? "none" : "grayscale(1)");
javascript:(function()%7Bst%20%3D%20!window.st%3Bdocument.querySelector(%22html%22).style.setProperty(%22filter%22%2C%20st%20%3F%20%22none%22%20%3A%20%22grayscale(1)%22)%7D)()
colour pls via https://mrcoles.com/bookmarklet/