darkmode.js

// js
const isDarkMode = (
	window.matchMedia &&
	window.matchMedia("(prefers-color-scheme: dark)").matches
)

window
	.matchMedia("(prefers-color-scheme: dark)")
	.addEventListener("change", e => {
	    const colorScheme = e.matches ? "dark" : "light"
	})

// css
@media (prefers-color-scheme: dark) {
	background-color: black;
	color: white;
}

@media (prefers-color-scheme: light) {
	background-color: white;
	color: black;
}