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HTML

              
                <h1>ダークモード / ライトモードの切り替え (Local Storage に保持)</h1>

<ul>
<li>デフォルトでは、デバイス (OS) の設定に連動して、ダークモード / ライトモードが自動的に設定されます。</li>
<li>ユーザーが任意で、ダークモード / ライトモードの設定を変更することができます。設定変更は Local Storage に保存されます。(つまり、セッションが終了しても、任意の設定変更は維持されます。)</li>
<li>後でユーザーが「ダークモード / ライトモードの設定はデバイス設定に連動させたい」と思ったときのために、リセットボタンも置いています。</li>
</ul>

<div role="group" aria-label="背景色の切り替え">
<button id="darkModeButton">ダークモード</button>
<button id="lightModeButton">ライトモード</button>
<button id="resetButton">OS の設定を適用</button>
</div>
              
            
!

CSS

              
                :root {
  --color-basic: #333333;
  --bgcolor-basic: #FFFFFF;
  --bgcolor-active: #DDDDDD;
}

@media (prefers-color-scheme: dark) {
  :root {
    --color-basic: #FFFFFF;
    --bgcolor-basic: #333333;
    --bgcolor-active: #666666;
  }
}

:root[data-theme='dark'] {
  --color-basic: #FFFFFF;
  --bgcolor-basic: #333333;
  --bgcolor-active: #666666;
}

:root[data-theme='light'] {
  --color-basic: #333333;
  --bgcolor-basic: #FFFFFF;
  --bgcolor-active: #DDDDDD;
}

body {
  color: var(--color-basic);
  background-color: var(--bgcolor-basic);
  font-family: sans-serif;
}

h1{font-size:large;}

li{font-size: small;}

button{
  -webkit-appearance: none;
  color: var(--color-basic);
  border: 2px var(--color-basic) solid;
  border-radius: 5px;
  padding: 0.5em;
  background-color: var(--bgcolor-basic);
  font-size: medium;
  margin-top: 0.5em;
}

button:focus{
  outline: 1px var(--color-basic) solid;
  outline-offset: 2px;
}

button.active{
  font-weight: bold;
  background-color: var(--bgcolor-active);
}


              
            
!

JS

              
                document.addEventListener('DOMContentLoaded', function() {
  const darkModeButton = document.getElementById('darkModeButton');
  const lightModeButton = document.getElementById('lightModeButton');
  const resetButton = document.getElementById('resetButton');

  const applyMode = (mode) => {
    if (mode === 'dark') {
      document.documentElement.setAttribute('data-theme', 'dark');
      setActiveButton(darkModeButton);
    } else if (mode === 'light') {
      document.documentElement.setAttribute('data-theme', 'light');
      setActiveButton(lightModeButton);
    } else {
      setActiveButton(resetButton);
    }
  };

  const setMode = (mode) => {
    localStorage.setItem('mode', mode);
    applyMode(mode);
  };

  const resetMode = () => {
    localStorage.removeItem('mode');
    applySystemMode();
  };

  const applySystemMode = () => {
    const mode = localStorage.getItem('mode');
    if (mode) {
      applyMode(mode);
    } else {
      const isDarkMode = window.matchMedia && window.matchMedia('(prefers-color-scheme: dark)').matches;
      if (isDarkMode) {
        document.documentElement.setAttribute('data-theme', 'dark');
      } else {
        document.documentElement.setAttribute('data-theme', 'light');
      }
      setActiveButton(resetButton); // システムデフォルトモードの場合に resetButton をアクティブにする
    }
  };

  const setActiveButton = (activeButton) => {
    darkModeButton.classList.remove('active');
    lightModeButton.classList.remove('active');
    resetButton.classList.remove('active');
    
    darkModeButton.setAttribute('aria-pressed', 'false');
    lightModeButton.setAttribute('aria-pressed', 'false');
    resetButton.setAttribute('aria-pressed', 'false');
    
    if (activeButton) {
      activeButton.classList.add('active');
      activeButton.setAttribute('aria-pressed', 'true');
    }
  };

  // ページロード時の初期モードを設定
  applySystemMode();

  darkModeButton.addEventListener('click', () => setMode('dark'));
  lightModeButton.addEventListener('click', () => setMode('light'));
  resetButton.addEventListener('click', resetMode);

  window.matchMedia('(prefers-color-scheme: dark)').addEventListener('change', function(e) {
    if (localStorage.getItem('mode')) return;
    applySystemMode();
  });
});
              
            
!
999px

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