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HTML

              
                <h1>System-Mirroring Dark Mode with Persisted Overrides</h1>

<p>This page will be light or dark based on the user’s system preferences until overridden with site-specific preference (either <code>light</code> or <code>dark</code>). CSS-wise, the change is made by adding a <code>theme--dark</code> class to <code>&lt;html&gt;</code>.</p>

<p>Locally saved user preference: <output class="badge" id="theme_preference" style="text-transform: uppercase">…<noscript> We really need JavaScript</noscript></output></p>

<p>
  <button id="light">Light</button>
  <button id="system">System</button>
  <button id="dark">Dark</button>
</p>

<p>JavaScript is required for dark mode, even though it can be done with pure CSS. (It is currently not possible to have manual overrides <em>and</em> system-mirroring dark mode without one of: JavaScript, duplicated CSS, or multiple stylesheets. If I am wrong, <a href="https://alanhogan.com/contact?from=https://codepen.io/alanhogan/pen/PoEgwxL&reason=removing%20duplication%20without%20losing%20CSS-only%20automatic%20dark%20mode">let me know</a>.)</p>

<p><a href="https://alanhogan.com/code/modern-html-template">Based on the modern HTML template</a> by <a href="https://alanhogan.com/">Alan Hogan</a></p>

<fieldset>
  <legend>Unstyled Elements</legend>
  <input type=checkbox />
  <input value="" />
  <input type=submit value=Go />
</fieldset>

<div class=scroll>
  This box is intended to
  show how scrollbars look,
  which may be affected by the
  <code>color-scheme</code>
  CSS property.
  <br>
  <br>
  Exciting, I know.
</diV>
              
            
!

CSS

              
                button.current {
  background-color: var(--page-bg-color);
  color: var(--text-color);
  border-color: var(--page-bg-color);
  border-style: solid;
  font-weight: 600;
  filter: contrast(0.8);
}
button {
  min-width: 6.5em;
  border-radius: 0.2em;
}
button:not(.current) {
  color: var(--page-bg-color);
  background-color: var(--link-color);
  border-color: var(--link-color);
}
button:not(:disabled) {
  cursor: pointer;
}

.scroll {
  margin: 1.6em 0;
  padding: 0.6em;
  border: 1px solid currentColor;
  max-width: 12em;
  max-height: 4em;
  overflow: auto;
}

/* Automatic dark mode, system fonts, and readable max-width
 * via https://ajh.us/template */

:root {
  --page-bg-color: white;
  --text-color: #222;
  --link-color: #06d;
  --visited-link-color: #91e;
  --code-color: #456658;
  --badge-bg: #eee;
  --badge-text-color: #444;
  color-scheme: light;
}

.theme--dark:root {
  --page-bg-color: #141414;
  --text-color: #eee;
  --link-color: #2af;
  --visited-link-color: #c5f;
  --code-color: #c5eddc;
  --badge-bg: #252525;
  --badge-text-color: #ccc;
  color-scheme: dark;
}

* {
  box-sizing: border-box;
}
html {
  margin: 0;
  padding: 0;
}
html,
body,
input,
button,
textarea {
  font-family: system-ui, sans-serif;
}
body {
  padding: calc(0.5em + 0.5vmin);
  margin: 0 auto;
  max-width: 40em;
  background-color: var(--page-bg-color);
  color: var(--text-color);
}

:link {
  color: var(--link-color);
}
:visited {
  color: var(--visited-link-color);
}

code,
tt,
kbd,
pre {
  font-family: "JetBrains Mono", "IBM Plex Mono", "Source Code Pro",
    "Cascadia Code", ui-monospace, Menlo, "Ubuntu Monospace", "Inconsolata",
    "Fira Code", "Deja Vu Sans Mono", "Bitstream Vera Sans Mono", Monaco,
    "Segoe UI Mono", "Roboto Mono", "Oxygen Mono", Consolas, monospace;
  font-size: 90%;
  color: var(--code-color);
}
pre code,
pre tt,
pre kbd {
  font-size: 100%;
}
.badge {
  font-size: 80%;
  font-weight: 500;
  display: inline-block;
  padding: 0.05em 0.4em;
  background: var(--badge-bg);
  color: var(--badge-text-color);
  border-radius: 0.2em;
  text-decoration: none;
  position: relative;
  bottom: 0.1em;
}

              
            
!

JS

              
                // Constants
const themes = { system: "system", dark: "dark", light: "light" };
const themeMediaQuery = matchMedia("(prefers-color-scheme: dark)");

// Helpers
const getThemePreference = () => localStorage.getItem("themePreference");

// State
const themeSettings = {
  system: themeMediaQuery.matches ? themes.dark : themes.light,
  preference: getThemePreference() || themes.system
};

function calculateTheme(settings) {
  if (settings.preference == themes.system) {
    return settings.system;
  } else if (settings.preference == themes.dark) {
    return themes.dark;
  } else return themes.light;
}
function applyTheme(theme) {
  if (theme === themes.dark) {
    document.documentElement.classList.add("theme--dark");
  } else {
    document.documentElement.classList.remove("theme--dark");
  }
}

function recalculateAndApplyTheme() {
  applyTheme(calculateTheme(themeSettings));
}

function initializeThemes() {
  recalculateAndApplyTheme();
  themeMediaQuery.addEventListener("change", (e) => {
    themeSettings.system = e.matches ? themes.dark : themes.light;
    recalculateAndApplyTheme();
  });
}

// Should run this in document.head() to avoid flashes of incorrect theme.
initializeThemes();

///////////////////////////////////////////////
/// BELOW THIS LINE CAN BE AFTER BODY/LOAD. ///
///////////////////////////////////////////////

// DOM HELPERS ----------
const qs = (s) => document.querySelector(s);

// USER PREFERENCE UPDATING -------
const setThemePreference = (pref) => {
  if (!themes.hasOwnProperty(pref)) {
    console.error("Invalid theme", pref, "not found in", themes.keys());
  } else {
    themeSettings.preference = pref;
    localStorage.setItem("themePreference", pref);
    recalculateAndApplyTheme();
  }
};

// HOOKING UP THE BUTTONS ----------
var showCurrentPreference = () => {
  const curPref = getThemePreference();
  qs("#theme_preference").innerText = curPref || "Never Set";
  const oldBtn = qs(".current");
  if (oldBtn) {
    oldBtn.classList.remove("current");
    oldBtn.disabled = false;
  }
  if (themes[curPref]) {
    const newBtn = qs(`#${curPref}`);
    newBtn.classList.add("current");
    newBtn.disabled = true;
  }
};

qs("#light").addEventListener("click", (e) => {
  setThemePreference(themes.light);
  showCurrentPreference();
});

qs("#system").addEventListener("click", (e) => {
  setThemePreference(themes.system);
  showCurrentPreference();
});

qs("#dark").addEventListener("click", (e) => {
  setThemePreference(themes.dark);
  showCurrentPreference();
});
// End button hook-ups

// Show current:
showCurrentPreference();

// TODO: Listen to event for theme preference changes to update the buttons.

              
            
!
999px

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