System, light, and dark themes

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<html lang="en">
  <head>
    <meta charset="utf-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1" />
    <meta
      http-equiv="Content-Security-Policy"
      content="default-src 'none'; style-src 'unsafe-inline'; script-src 'unsafe-inline'; img-src data:; connect-src 'none'; form-action 'none'; object-src 'none'; base-uri 'none'"
    />
    <title>System-aware theme switcher | MRHTML</title>
    <style>
      /* MRHTML standalone example. No external styles, fonts, or assets. */
      :root {
        color-scheme: light;
        --paper: #f5f3ee;
        --surface: #fff;
        --ink: #1e2b42;
        --muted: #59657a;
        --line: #d9dee7;
        --soft: #eef1fb;
        --blue: #344ed5;
        --blue-hover: #263caf;
        --button-text: #fff;
        --focus: #b65716;
        --error: #ad253d;
        font:
          16px/1.6 system-ui,
          -apple-system,
          BlinkMacSystemFont,
          'Segoe UI',
          sans-serif;
        color: var(--ink);
        background: var(--paper);
      }
      * {
        box-sizing: border-box;
      }
      body {
        margin: 0;
        min-height: 100svh;
        display: grid;
        place-items: center;
        padding: 40px 24px;
      }
      main {
        width: min(680px, 100%);
        min-width: 0;
      }
      h1,
      h2,
      h3,
      p {
        margin: 0;
      }
      h1,
      h2,
      h3 {
        line-height: 1.15;
        letter-spacing: -0.045em;
      }
      h1 {
        font-size: clamp(1.9rem, 5vw, 2.65rem);
        font-weight: 650;
        max-width: 570px;
      }
      h2 {
        font-size: 1.35rem;
        font-weight: 650;
      }
      p {
        color: var(--muted);
      }
      button,
      input,
      textarea {
        font: inherit;
      }
      button {
        cursor: pointer;
      }
      button:disabled {
        cursor: not-allowed;
        opacity: 0.65;
      }
      button,
      a,
      input,
      textarea {
        -webkit-tap-highlight-color: transparent;
      }
      :focus-visible {
        outline: 3px solid var(--focus);
        outline-offset: 4px;
      }
      .eyebrow {
        display: flex;
        align-items: center;
        gap: 10px;
        color: var(--muted);
        font-size: 0.65rem;
        font-weight: 650;
        letter-spacing: 0.13em;
        text-transform: uppercase;
        margin-bottom: 22px;
      }
      .eyebrow::before {
        content: '';
        width: 8px;
        height: 8px;
        background: var(--blue);
        border-radius: 50%;
      }
      .intro {
        font-size: 0.94rem;
        max-width: 540px;
        margin-top: 17px;
      }
      .card {
        margin-top: 28px;
        padding: 28px;
        background: var(--surface);
        border: 1px solid var(--line);
        border-radius: 16px;
        box-shadow: 0 8px 28px #14264a05;
      }
      .card-label {
        color: var(--muted);
        font-size: 0.62rem;
        font-weight: 650;
        letter-spacing: 0.11em;
        text-transform: uppercase;
        margin-bottom: 11px;
      }
      .button {
        display: inline-flex;
        align-items: center;
        justify-content: center;
        gap: 13px;
        padding: 12px 19px;
        min-height: 46px;
        border: 1px solid transparent;
        border-radius: 8px;
        background: var(--blue);
        color: var(--button-text);
        font-size: 0.85rem;
        font-weight: 600;
        line-height: 1.45;
        transition:
          background 0.15s,
          transform 0.15s;
      }
      .button:hover:not(:disabled) {
        background: var(--blue-hover);
        transform: translateY(-1px);
      }
      .button.secondary {
        background: var(--surface);
        border-color: var(--line);
        color: var(--ink);
      }
      .button.secondary:hover:not(:disabled) {
        background: var(--soft);
      }
      .button svg {
        display: block;
        width: 18px;
        height: 18px;
        flex: none;
      }
      .hint {
        font-size: 0.74rem;
        line-height: 1.75;
        margin-top: 20px;
        max-width: 570px;
      }
      .status {
        min-height: 1.6em;
        margin-top: 17px;
        font-size: 0.8rem;
        color: var(--ink);
        overflow-wrap: anywhere;
      }
      .actions {
        display: flex;
        align-items: center;
        flex-wrap: wrap;
        gap: 12px;
      }
      [hidden] {
        display: none !important;
      }
      @media (max-width: 430px) {
        body {
          padding: 30px 18px;
        }
        .card {
          padding: 23px 20px;
        }
        .eyebrow {
          font-size: 0.6rem;
          letter-spacing: 0.11em;
        }
        .intro {
          font-size: 0.88rem;
        }
      }
      @media (prefers-reduced-motion: reduce) {
        *,
        *::before,
        *::after {
          animation: none !important;
          transition: none !important;
        }
        .button:hover:not(:disabled) {
          transform: none;
        }
      }

      :root[data-theme='dark'] {
        color-scheme: dark;
        --paper: #141d2d;
        --surface: #1e2a3e;
        --ink: #eef2fb;
        --muted: #bdc9db;
        --line: #45536b;
        --soft: #293752;
        --blue: #a7b6ff;
        --blue-hover: #c1ccff;
        --button-text: #172557;
        --focus: #f2be77;
      }
      /* Without JavaScript, the page still follows the device's dark preference. */
      @media (prefers-color-scheme: dark) {
        :root:not([data-theme]) {
          color-scheme: dark;
          --paper: #141d2d;
          --surface: #1e2a3e;
          --ink: #eef2fb;
          --muted: #bdc9db;
          --line: #45536b;
          --soft: #293752;
          --blue: #a7b6ff;
          --blue-hover: #c1ccff;
          --button-text: #172557;
          --focus: #f2be77;
        }
      }
      .theme-controls {
        display: flex;
        flex-wrap: wrap;
        gap: 8px;
        padding: 7px;
        margin: 25px 0 0;
        border: 1px solid var(--line);
        background: var(--surface);
        border-radius: 12px;
        width: fit-content;
        max-width: 100%;
      }
      .theme-controls legend {
        position: absolute;
        width: 1px;
        height: 1px;
        margin: -1px;
        overflow: hidden;
        clip: rect(0, 0, 0, 0);
        white-space: nowrap;
      }
      .theme-choice {
        position: relative;
        cursor: pointer;
      }
      .theme-choice input {
        position: absolute;
        margin: 0;
        width: 100%;
        height: 100%;
        inset: 0;
        opacity: 0;
        cursor: pointer;
      }
      .theme-choice > span {
        display: flex;
        align-items: center;
        justify-content: center;
        gap: 8px;
        min-height: 44px;
        padding: 10px 17px;
        font-size: 0.8rem;
        border: 1px solid transparent;
        border-radius: 7px;
        color: var(--muted);
      }
      .theme-choice input:checked + span {
        color: var(--ink);
        background: var(--soft);
        border-color: var(--line);
      }
      .theme-choice input:focus-visible + span {
        outline: 3px solid var(--focus);
        outline-offset: 2px;
      }
      .theme-choice input:disabled + span {
        opacity: 0.6;
      }
      .theme-choice .choice-symbol {
        font-size: 17px;
        line-height: 1;
      }
      .reading-card {
        margin-top: 24px;
      }
      .reading-toolbar {
        display: flex;
        align-items: center;
        justify-content: space-between;
        gap: 15px;
        border-bottom: 1px solid var(--line);
        padding-bottom: 18px;
        margin-bottom: 23px;
      }
      .reading-toolbar > span {
        font-size: 0.67rem;
        color: var(--muted);
      }
      .window-dots {
        display: flex;
        gap: 5px;
      }
      .window-dots span {
        width: 6px;
        height: 6px;
        background: var(--muted);
        border-radius: 50%;
      }
      .reading-card h2 {
        font-size: 1.65rem;
        max-width: 420px;
      }
      .reading-card > p {
        font-size: 0.86rem;
        max-width: 460px;
        margin-top: 16px;
      }
      .theme-footer {
        margin-top: 25px;
        display: flex;
        align-items: center;
        flex-wrap: wrap;
        gap: 10px 15px;
      }
      .theme-badge {
        display: inline-flex;
        align-items: center;
        gap: 7px;
        color: var(--ink);
        padding: 5px 10px;
        border: 1px solid var(--line);
        border-radius: 30px;
        font-size: 0.67rem;
        background: var(--soft);
      }
      .theme-badge::before {
        content: '';
        width: 6px;
        height: 6px;
        border-radius: 50%;
        background: var(--blue);
      }
      .theme-footer > span:last-child {
        font-size: 0.67rem;
        color: var(--muted);
      }
      @media (max-width: 430px) {
        .theme-choice > span {
          padding-inline: 12px;
          font-size: 0.75rem;
        }
        .theme-controls {
          gap: 4px;
          padding: 5px;
        }
        .reading-card h2 {
          font-size: 1.5rem;
        }
      }
    </style>
  </head>
  <body>
    <main>
      <p class="eyebrow">MRHTML / Color preferences</p>
      <h1>A little more comfortable.<br />In any light.</h1>
      <p class="intro">
        Follow your device's appearance, or choose the theme that feels right
        for you.
      </p>
      <fieldset class="theme-controls" id="theme-controls" disabled>
        <legend>Appearance</legend>
        <label class="theme-choice"
          ><input type="radio" name="theme" value="system" checked /><span
            ><span class="choice-symbol" aria-hidden="true">◐</span>System</span
          ></label
        ><label class="theme-choice"
          ><input type="radio" name="theme" value="light" /><span
            ><span class="choice-symbol" aria-hidden="true">☀</span>Light</span
          ></label
        ><label class="theme-choice"
          ><input type="radio" name="theme" value="dark" /><span
            ><span class="choice-symbol" aria-hidden="true">☾</span>Dark</span
          ></label
        >
      </fieldset>
      <section class="card reading-card" aria-labelledby="reading-heading">
        <div class="reading-toolbar">
          <div class="window-dots" aria-hidden="true">
            <span></span><span></span><span></span>
          </div>
          <span>MRHTML / Interface notes</span>
        </div>
        <p class="card-label">Make space for the reader</p>
        <h2 id="reading-heading">
          Good design leaves room<br />for a personal preference.
        </h2>
        <p>
          A thoughtful interface works with the way people read. Clear contrast,
          a familiar choice, and a setting that follows them through the day.
        </p>
        <div class="theme-footer">
          <span class="theme-badge" id="current-theme">System theme</span
          ><span>Same content. A different light.</span>
        </div>
      </section>
      <p class="status" id="theme-status" role="status"></p>
      <p class="hint" id="storage-note">
        Your choice can be remembered when browser storage is available.
      </p>
      <noscript
        ><p class="hint">
          JavaScript is off. The page follows your device theme, but the
          appearance controls are unavailable.
        </p></noscript
      >
    </main>

    <script>
      // MRHTML theme preference: storage is optional, including in sandboxed previews.
      const storageKey = 'mrhtml-snippet-theme';
      const allowedThemes = ['system', 'light', 'dark'];
      const systemTheme = window.matchMedia('(prefers-color-scheme: dark)');
      const controls = document.getElementById('theme-controls');
      const status = document.getElementById('theme-status');
      const storageNote = document.getElementById('storage-note');
      let preference = 'system';
      try {
        const saved = localStorage.getItem(storageKey);
        if (allowedThemes.includes(saved)) {
          preference = saved;
          storageNote.textContent =
            'Saved preference restored from this browser.';
        }
      } catch {
        storageNote.textContent =
          'Browser storage is unavailable. Your choice still works on this page.';
      }
      function applyTheme() {
        const resolved =
          preference === 'system'
            ? systemTheme.matches
              ? 'dark'
              : 'light'
            : preference;
        document.documentElement.dataset.theme = resolved;
        document.getElementById('current-theme').textContent =
          `${resolved === 'dark' ? 'Dark' : 'Light'} theme`;
        document.querySelectorAll('input[name="theme"]').forEach((input) => {
          input.checked = input.value === preference;
        });
        status.textContent =
          preference === 'system'
            ? `Following your system: ${resolved} theme.`
            : `${resolved === 'dark' ? 'Dark' : 'Light'} theme selected.`;
      }
      controls.addEventListener('change', (event) => {
        const value = event.target.value;
        if (!allowedThemes.includes(value)) return;
        preference = value;
        applyTheme();
        try {
          localStorage.setItem(storageKey, preference);
          storageNote.textContent = 'Preference saved in this browser.';
        } catch {
          storageNote.textContent =
            'Browser storage is unavailable. Your choice still works on this page.';
        }
      });
      systemTheme.addEventListener('change', () => {
        if (preference === 'system') applyTheme();
      });
      applyTheme();
      controls.disabled = false;
    </script>
  </body>
</html>