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How to trigger a dark theme at night

I recently added some JavaScript to auto-trigger a dark theme on this site at night. The strategy is simple: find the user’s current hour, and then place a dark class on the <html> element if it’s late at night. This is the bare minimum JavaScript necessary to do this:

var hour = new Date().getHours()
var isNight = hour < 6 || hour > 21
if (isNight) {
var html = document.getElementsByTagName('html')[0]

Then, you can make your CSS respond appropriately:

/* Make the page dark */
html.dark {
background: #000;
color: #fff;

/* Adjust elements inside the page if needed */
.dark .element {
background: #0a0a0a;

For best results, make sure that your dark CSS comes at the end of your normal CSS (so it takes precedence); otherwise, you might need lots of !important overrides.

You can also let your users toggle the theme themselves. After adding your CSS, just create a button that runs this function:

function toggleDarkTheme() {
var html = document.getElementsByTagName('html')[0]

And that’s that! Happy coding.