Accessible Dark/Light Toggle

*,
*::before,
*::after {
  box-sizing: border-box;
  margin: 0;
  padding: 0;
}

/* ── Dark theme (default) ── */
:root {
  color-scheme: dark;
  --bg: #0a0a0a;
  --surface: #141414;
  --elevated: #1a1a1a;
  --border: #262626;
  --text: #e5e5e5;
  --text-secondary: #a3a3a3;
  --text-muted: #737373;
  --link: #60a5fa;
  --accent: #3b82f6;
  --accent-text: #ffffff;
  --input-bg: #171717;
  --input-border: #333333;
  --input-text: #e5e5e5;
  --btn-outline-border: #404040;
  --transition-speed: 0.3s;
}

/* ── Light theme ── */
.light {
  color-scheme: light;
  --bg: #ffffff;
  --surface: #f5f5f5;
  --elevated: #fafafa;
  --border: #e5e5e5;
  --text: #111111;
  --text-secondary: #525252;
  --text-muted: #737373;
  --link: #2563eb;
  --accent: #2563eb;
  --accent-text: #ffffff;
  --input-bg: #ffffff;
  --input-border: #d4d4d4;
  --input-text: #111111;
  --btn-outline-border: #d4d4d4;
}

body {
  font-family: Inter, system-ui, -apple-system, sans-serif;
  background: var(--bg);
  color: var(--text);
  min-height: 100vh;
  display: flex;
  justify-content: center;
  padding: 0;
  transition: background var(--transition-speed), color var(--transition-speed);
}

.page {
  width: 100%;
  max-width: 760px;
  padding: 1.5rem 2rem;
}

/* ── Header ── */
.header {
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding-bottom: 1rem;
  border-bottom: 1px solid var(--border);
  margin-bottom: 2rem;
  transition: border-color var(--transition-speed);
}

.header-left {
  display: flex;
  align-items: center;
  gap: 2rem;
}

.logo {
  font-size: 1.125rem;
  font-weight: 800;
  letter-spacing: -0.02em;
  white-space: nowrap;
}

.nav {
  display: flex;
  gap: 0.25rem;
}

.nav-link {
  color: var(--text-secondary);
  text-decoration: none;
  padding: 0.375rem 0.75rem;
  border-radius: 0.375rem;
  font-size: 0.8125rem;
  font-weight: 500;
  transition: background 0.15s, color 0.15s;
}

.nav-link:hover {
  background: var(--surface);
  color: var(--text);
}
.nav-link.active {
  color: var(--text);
  font-weight: 600;
}

/* ── Theme toggle button ── */
.theme-btn {
  display: flex;
  align-items: center;
  gap: 0.5rem;
  background: var(--surface);
  border: 1px solid var(--border);
  border-radius: 0.5rem;
  color: var(--text);
  padding: 0.5rem 0.875rem;
  font-size: 0.8125rem;
  font-weight: 600;
  cursor: pointer;
  transition: background 0.15s, border-color 0.15s;
}

.theme-btn:hover {
  border-color: var(--text-muted);
}

/* Show/hide sun/moon icons */
.icon-sun {
  display: none;
}
.icon-moon {
  display: block;
}
.light .icon-sun {
  display: block;
}
.light .icon-moon {
  display: none;
}

/* ── Hero ── */
.hero {
  margin-bottom: 2rem;
  padding: 2rem;
  background: var(--surface);
  border: 1px solid var(--border);
  border-radius: 0.75rem;
  transition: background var(--transition-speed), border-color var(--transition-speed);
}

.hero-title {
  font-size: 1.5rem;
  font-weight: 800;
  margin-bottom: 0.5rem;
}
.hero-text {
  color: var(--text-secondary);
  font-size: 0.9375rem;
  line-height: 1.6;
}

/* ── Cards ── */
.cards {
  display: grid;
  grid-template-columns: repeat(3, 1fr);
  gap: 0.75rem;
  margin-bottom: 2rem;
}

.card {
  background: var(--surface);
  border: 1px solid var(--border);
  border-radius: 0.75rem;
  padding: 1.25rem;
  transition: background var(--transition-speed), border-color var(--transition-speed);
}

.card-header {
  display: flex;
  align-items: flex-start;
  justify-content: space-between;
  margin-bottom: 0.75rem;
}

.card-title {
  font-size: 0.875rem;
  font-weight: 700;
}

.card-body {
  font-size: 0.8125rem;
  color: var(--text-secondary);
  line-height: 1.6;
}

.card-body.secondary {
  color: var(--text-secondary);
}

/* ── Ratio badges ── */
.ratio-badge {
  font-size: 0.625rem;
  font-weight: 700;
  font-family: "SF Mono", "Fira Code", monospace;
  padding: 0.125rem 0.375rem;
  border-radius: 0.25rem;
  background: var(--elevated);
  border: 1px solid var(--border);
  white-space: nowrap;
  transition: background var(--transition-speed), border-color var(--transition-speed);
}

.ratio-badge.small {
  font-size: 0.5625rem;
  margin-top: 0.25rem;
  display: inline-block;
}
.ratio-badge.inline {
  vertical-align: baseline;
}

/* ── Form ── */
.form-section {
  background: var(--surface);
  border: 1px solid var(--border);
  border-radius: 0.75rem;
  padding: 1.5rem;
  margin-bottom: 2rem;
  transition: background var(--transition-speed), border-color var(--transition-speed);
}

.section-title {
  font-size: 1rem;
  font-weight: 700;
  margin-bottom: 1rem;
}

.form-grid {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: 1rem;
  margin-bottom: 1.25rem;
}

.form-group label {
  display: block;
  font-size: 0.75rem;
  font-weight: 600;
  color: var(--text-secondary);
  margin-bottom: 0.375rem;
}

.form-group input {
  width: 100%;
  background: var(--input-bg);
  border: 1px solid var(--input-border);
  border-radius: 0.5rem;
  color: var(--input-text);
  padding: 0.5rem 0.75rem;
  font-size: 0.875rem;
  outline: none;
  transition: border-color 0.15s, background var(--transition-speed);
}

.form-group input:focus {
  border-color: var(--accent);
}

.form-actions {
  display: flex;
  gap: 0.5rem;
}

.btn {
  padding: 0.5rem 1rem;
  border-radius: 0.5rem;
  font-size: 0.8125rem;
  font-weight: 600;
  cursor: pointer;
  border: 1px solid transparent;
  transition: background 0.15s, border-color 0.15s;
}

.btn-primary {
  background: var(--accent);
  color: var(--accent-text);
}
.btn-primary:hover {
  opacity: 0.9;
}
.btn-outline {
  background: transparent;
  color: var(--text);
  border-color: var(--btn-outline-border);
}
.btn-outline:hover {
  background: var(--surface);
}

/* ── Text section ── */
.text-section {
  margin-bottom: 2rem;
}

.body-text {
  font-size: 0.9375rem;
  line-height: 1.7;
  color: var(--text-secondary);
}

.body-text a {
  color: var(--link);
  text-decoration: underline;
  text-underline-offset: 2px;
}
.body-text a:hover {
  opacity: 0.8;
}

/* ── Contrast table ── */
.contrast-table-section {
  margin-bottom: 2rem;
}

.contrast-table {
  width: 100%;
  border-collapse: collapse;
  font-size: 0.8125rem;
}

.contrast-table th {
  text-align: left;
  font-size: 0.6875rem;
  font-weight: 600;
  text-transform: uppercase;
  letter-spacing: 0.06em;
  color: var(--text-muted);
  padding: 0.625rem 0.75rem;
  border-bottom: 1px solid var(--border);
}

.contrast-table td {
  padding: 0.625rem 0.75rem;
  border-bottom: 1px solid var(--border);
  transition: border-color var(--transition-speed);
}

.contrast-table .swatch-cell {
  width: 24px;
  height: 24px;
  border-radius: 0.25rem;
  border: 1px solid var(--border);
  display: inline-block;
  vertical-align: middle;
}

.pass-cell {
  color: #22c55e;
  font-weight: 700;
}
.fail-cell {
  color: #ef4444;
  font-weight: 700;
}

@media (max-width: 640px) {
  .header {
    flex-direction: column;
    align-items: flex-start;
    gap: 0.75rem;
  }
  .header-left {
    flex-direction: column;
    align-items: flex-start;
    gap: 0.5rem;
  }
  .cards {
    grid-template-columns: 1fr;
  }
  .form-grid {
    grid-template-columns: 1fr;
  }
}

(function () {
  var toggleBtn = document.getElementById("theme-toggle");
  var themeLabel = document.getElementById("theme-label");
  var tbody = document.getElementById("contrast-tbody");
  var body = document.body;

  // ── WCAG contrast helpers ──
  function hexToRgb(hex) {
    hex = hex.replace("#", "");
    if (hex.length === 3) hex = hex[0] + hex[0] + hex[1] + hex[1] + hex[2] + hex[2];
    var n = parseInt(hex, 16);
    return [(n >> 16) & 255, (n >> 8) & 255, n & 255];
  }

  function linearize(c) {
    var s = c / 255;
    return s <= 0.04045 ? s / 12.92 : Math.pow((s + 0.055) / 1.055, 2.4);
  }

  function luminance(rgb) {
    return 0.2126 * linearize(rgb[0]) + 0.7152 * linearize(rgb[1]) + 0.0722 * linearize(rgb[2]);
  }

  function contrastRatio(hex1, hex2) {
    var l1 = luminance(hexToRgb(hex1));
    var l2 = luminance(hexToRgb(hex2));
    return (Math.max(l1, l2) + 0.05) / (Math.min(l1, l2) + 0.05);
  }

  // ── Theme definitions ──
  var themes = {
    dark: {
      bg: "#0a0a0a",
      surface: "#141414",
      text: "#e5e5e5",
      textSecondary: "#a3a3a3",
      link: "#60a5fa",
      inputBg: "#171717",
      inputText: "#e5e5e5",
    },
    light: {
      bg: "#ffffff",
      surface: "#f5f5f5",
      text: "#111111",
      textSecondary: "#525252",
      link: "#2563eb",
      inputBg: "#ffffff",
      inputText: "#111111",
    },
  };

  var colorPairs = [
    { name: "Text / Background", fgKey: "text", bgKey: "bg", pairId: "text-bg" },
    { name: "Text / Surface", fgKey: "text", bgKey: "surface", pairId: "text-surface" },
    { name: "Secondary / Background", fgKey: "textSecondary", bgKey: "bg", pairId: "secondary-bg" },
    { name: "Link / Background", fgKey: "link", bgKey: "bg", pairId: "link-bg" },
    { name: "Input text / Input bg", fgKey: "inputText", bgKey: "inputBg", pairId: "input-text" },
  ];

  var currentTheme = "dark";

  // ── Detect OS preference ──
  function getPreferred() {
    var stored = localStorage.getItem("theme-preference");
    if (stored) return stored;
    if (window.matchMedia && window.matchMedia("(prefers-color-scheme: light)").matches)
      return "light";
    return "dark";
  }

  // ── Apply theme ──
  function applyTheme(theme) {
    currentTheme = theme;
    if (theme === "light") {
      body.classList.add("light");
    } else {
      body.classList.remove("light");
    }
    themeLabel.textContent = theme === "light" ? "Dark" : "Light";
    localStorage.setItem("theme-preference", theme);
    updateRatios();
  }

  // ── Update ratio badges and table ──
  function updateRatios() {
    var t = themes[currentTheme];

    // Update inline badges
    var badges = document.querySelectorAll(".ratio-badge[data-pair]");
    badges.forEach(function (badge) {
      var pair = colorPairs.find(function (p) {
        return p.pairId === badge.getAttribute("data-pair");
      });
      if (pair) {
        var ratio = contrastRatio(t[pair.fgKey], t[pair.bgKey]);
        badge.textContent = ratio.toFixed(1) + ":1";
        badge.style.color = ratio >= 4.5 ? "#22c55e" : "#ef4444";
      }
    });

    // Update table
    tbody.innerHTML = "";
    colorPairs.forEach(function (pair) {
      var fg = t[pair.fgKey];
      var bg = t[pair.bgKey];
      var ratio = contrastRatio(fg, bg);
      var passAA = ratio >= 4.5;
      var passAAA = ratio >= 7;

      var tr = document.createElement("tr");
      tr.innerHTML =
        "<td>" +
        pair.name +
        "</td>" +
        '<td><span class="swatch-cell" style="background:' +
        fg +
        '"></span> ' +
        fg +
        "</td>" +
        '<td><span class="swatch-cell" style="background:' +
        bg +
        '"></span> ' +
        bg +
        "</td>" +
        "<td><strong>" +
        ratio.toFixed(2) +
        ":1</strong></td>" +
        '<td class="' +
        (passAA ? "pass-cell" : "fail-cell") +
        '">' +
        (passAA ? "\u2713 Pass" : "\u2717 Fail") +
        "</td>" +
        '<td class="' +
        (passAAA ? "pass-cell" : "fail-cell") +
        '">' +
        (passAAA ? "\u2713 Pass" : "\u2717 Fail") +
        "</td>";
      tbody.appendChild(tr);
    });
  }

  // ── Toggle ──
  toggleBtn.addEventListener("click", function () {
    applyTheme(currentTheme === "dark" ? "light" : "dark");
  });

  // ── Listen for OS changes ──
  if (window.matchMedia) {
    window.matchMedia("(prefers-color-scheme: light)").addEventListener("change", function (e) {
      if (!localStorage.getItem("theme-preference")) {
        applyTheme(e.matches ? "light" : "dark");
      }
    });
  }

  // ── Init ──
  applyTheme(getPreferred());
})();

<!doctype html>
<html lang="en">
<head>
  <meta charset="UTF-8" />
  <meta name="viewport" content="width=device-width, initial-scale=1.0" />
  <title>Accessible Dark/Light Toggle</title>
  <link rel="stylesheet" href="style.css" />
</head>
<body>
  <div class="page">
    <!-- Header -->
    <header class="header">
      <div class="header-left">
        <h1 class="logo">Accessible Themes</h1>
        <nav class="nav" aria-label="Main">
          <a href="#" class="nav-link active" onclick="return false">Home</a>
          <a href="#" class="nav-link" onclick="return false">Docs</a>
          <a href="#" class="nav-link" onclick="return false">API</a>
          <a href="#" class="nav-link" onclick="return false">Blog</a>
        </nav>
      </div>
      <button class="theme-btn" id="theme-toggle" aria-label="Toggle dark/light mode">
        <svg class="icon-sun" width="18" height="18" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"><circle cx="12" cy="12" r="5"/><line x1="12" y1="1" x2="12" y2="3"/><line x1="12" y1="21" x2="12" y2="23"/><line x1="4.22" y1="4.22" x2="5.64" y2="5.64"/><line x1="18.36" y1="18.36" x2="19.78" y2="19.78"/><line x1="1" y1="12" x2="3" y2="12"/><line x1="21" y1="12" x2="23" y2="12"/><line x1="4.22" y1="19.78" x2="5.64" y2="18.36"/><line x1="18.36" y1="5.64" x2="19.78" y2="4.22"/></svg>
        <svg class="icon-moon" width="18" height="18" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"><path d="M21 12.79A9 9 0 1 1 11.21 3 7 7 0 0 0 21 12.79z"/></svg>
        <span class="theme-label" id="theme-label">Light</span>
      </button>
    </header>

    <!-- Hero -->
    <section class="hero">
      <h2 class="hero-title">WCAG-Compliant Themes</h2>
      <p class="hero-text">Both dark and light themes maintain minimum 4.5:1 contrast ratio for all text elements and interactive controls.</p>
    </section>

    <!-- Cards with contrast ratios -->
    <div class="cards">
      <div class="card">
        <div class="card-header">
          <h3 class="card-title">Text on Background</h3>
          <span class="ratio-badge" data-pair="text-bg"></span>
        </div>
        <p class="card-body">Primary text content displayed on the main background color. This combination exceeds AA requirements in both themes.</p>
      </div>
      <div class="card">
        <div class="card-header">
          <h3 class="card-title">Text on Surface</h3>
          <span class="ratio-badge" data-pair="text-surface"></span>
        </div>
        <p class="card-body">Content placed on elevated surfaces like cards and panels. Contrast ratios are validated for both themes.</p>
      </div>
      <div class="card">
        <div class="card-header">
          <h3 class="card-title">Secondary Text</h3>
          <span class="ratio-badge" data-pair="secondary-bg"></span>
        </div>
        <p class="card-body secondary">Secondary text uses a muted color while still maintaining the minimum 4.5:1 contrast ratio required by WCAG AA.</p>
      </div>
    </div>

    <!-- Form -->
    <div class="form-section">
      <h3 class="section-title">Form Controls</h3>
      <div class="form-grid">
        <div class="form-group">
          <label for="input-name">Name</label>
          <input type="text" id="input-name" value="Jane Doe" />
          <span class="ratio-badge small" data-pair="input-text"></span>
        </div>
        <div class="form-group">
          <label for="input-email">Email</label>
          <input type="email" id="input-email" value="[email protected]" />
          <span class="ratio-badge small" data-pair="input-text"></span>
        </div>
      </div>
      <div class="form-actions">
        <button class="btn btn-primary">Submit</button>
        <button class="btn btn-outline">Cancel</button>
      </div>
    </div>

    <!-- Inline text -->
    <div class="text-section">
      <h3 class="section-title">Inline Elements</h3>
      <p class="body-text">
        Regular text with <a href="#" onclick="return false">accessible links</a> that remain
        distinguishable in both themes. Links maintain a <span class="ratio-badge inline" data-pair="link-bg"></span>
        contrast ratio. <strong>Bold text</strong> and <em>emphasized text</em> inherit the primary
        color and its high contrast ratio.
      </p>
    </div>

    <!-- Contrast table -->
    <div class="contrast-table-section">
      <h3 class="section-title">Contrast Ratios</h3>
      <table class="contrast-table" aria-label="Contrast ratios for current theme">
        <thead>
          <tr>
            <th>Pair</th>
            <th>Foreground</th>
            <th>Background</th>
            <th>Ratio</th>
            <th>AA</th>
            <th>AAA</th>
          </tr>
        </thead>
        <tbody id="contrast-tbody"></tbody>
      </table>
    </div>
  </div>
  <script src="script.js"></script>
</body>
</html>