Lightswitch - adding dark mode compatibility to a website

One of my goals developing this blog was to design it to work with both light and dark color schemes. Having different background colors is very helpful for readability and eyestrain both during the day and night, and follows the ongoing trend of computers adapting to blending into to their surroundings.

Dark mode was the major user-facing feature in September's release of MacOS Mojave and has been well received, which means other platforms (including iOS) will surely follow. It was only a matter of time before this was brought to the web, and indeed that happened with last week's release of Safari Technology Preview 68, which lets websites specify different styles based on whether the OS is in light mode or dark mode (technically, the prefers-color-scheme media query).

However, there is one issue with just letting OS mode determine a website's color scheme - user preference. Because OS mode doesn't change based on day/night, users are going to set their OS mode once and probably leave just it that way, regardless of time of day. Those users may not always want a website to be light or dark based on their OS, and may wish to override the default.

Lightswitch.js active on this website

My solution is a bit javascript I call Lightswitch that automatically detects the user's OS mode, and also allows the user to override that mode with a button you put on your site. On this blog, the button is the half circle icon in the top right corner, but you may attach the override functionality anywhere on your site -- such as this link.

Here's the code:

/*******************************************************************************
LIGHTSWITCH: A DARK MODE SWITCHER WITH USER OVERRIDE
By Nick Punt 10/26/2018
How to use:
  *  Create two color schemes in CSS under the classes 'light' and 'dark'
  *  Add the class 'light' or 'dark' to your body as your default color scheme
  *  Add button to page with id 'lightswitch', which lets users change/override
  *  Use the class 'flipswitch' for any style changes you want on lightswitch
Logic:
  1. When user hits page for first time, color scheme is based on OS/browser
     (if supported), otherwise it defaults to the body class you added
  2. When user clicks lightswitch to override colors, their preference is stored
  3. When user alters their OS light/dark mode, switch to dark if dark mode,
     and light if light mode
     
Note:
The 'prefers-color-scheme' css support is currently only available in Safari 
Technology Preview 68+. 
*******************************************************************************/

// New prefers-color-scheme media query to detect OS light/dark mode setting
var prefers_light = window.matchMedia('(prefers-color-scheme: light)')
var prefers_dark = window.matchMedia('(prefers-color-scheme: dark)')

// Change to dark and rotate the switch icon
function darkmode() {
  document.body.classList.replace('light', 'dark');
  document.getElementById("nav-light").classList.add("flipswitch");
}

// Change to light and rotate the switch icon
function lightmode() {
  document.body.classList.replace('dark', 'light');
  document.getElementById("nav-light").classList.remove("flipswitch");
}

// Initialization triggers light/dark mode based on prior preference, then OS setting
if(localStorage.getItem("mode")=="dark") {
  darkmode();
} else if(localStorage.getItem("mode")=="light") {
  lightmode();
} else if(prefers_light.matches) {
  lightmode();
} else if(prefers_dark.matches) {
  darkmode();
}

// Fires when user clicks light/dark mode switch in top right
function handleThemeUpdate() {
  if (document.body.classList.contains('light')) {
    darkmode();
    localStorage.setItem("mode", "dark");
  } else {
    lightmode();
    localStorage.setItem("mode", "light");
  }
}

// Runs when OS changes light/dark mode. Changes only if you were on default
// color state (light on light mode, dark on dark mode).
function OSColorChange() {
  if (prefers_light.matches) {
    lightmode();
    localStorage.setItem("mode", "light");
  } else if (prefers_dark.matches) {
    darkmode();
    localStorage.setItem("mode", "dark");
  }
}

// Listeners for when you change OS setting for light/dark mode
prefers_light.addListener(OSColorChange)
prefers_dark.addListener(OSColorChange)

Download on Github

The only known issue with Lightswitch currently is that since the site defaults to light, when a user loads a page with a lot of content (e.g. the blog index), the screen may flash to the light background briefly before the javascript is run.

Setting content to light or dark based on the OS mode is a first step to having our computers be truly responsive to our surroundings, but it's not the final word. Ideally, websites and documents would set a color scheme based on environmental factors like time of day and lighting in the room. Javascript isn't the right way to solve these problems though - it's the OS' job to factor these in to provide a consistent experience from UI to apps to content. The best we can do in the meantime is to set up our content with light and dark variants and allow users to set override preferences on this content.


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