Detect dark mode in browser and colour website accordingly.

[RokeJulianLockhart]

https://stackoverflow.com/a/57795518/9731176

If you want to detect it from JS, you can use this code:

if (window.matchMedia && window.matchMedia('(prefers-color-scheme: dark)').matches) {
    // dark mode
}

To watch for changes:

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

If implemented, allow the user to force browser theme preference over #231 (comment).