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HTML 

              
                <!--------- jsの読み込み --------->
<script src="https://code.jquery.com/jquery-3.6.0.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery-cookie/1.4.1/jquery.cookie.js"></script>

<!--------- 文字サイズ変更ボタン --------->
<div class="btn_wrap">
    <span>文字の大きさ</span>
    <button type="button" class="sizeBtn" id="font-small">小</button>
    <button type="button" class="sizeBtn is_active" id="font-middle">中</button>
    <button type="button" class="sizeBtn" id="font-large">大</button>
</div>
<!--------- 背景色変更ボタン --------->
<div class="btn_wrap">
    <span>背景の色</span>
    <button type="button" class="bgBtn is_active" id="bg-white">白</button>
    <button type="button" class="bgBtn" id="bg-blue">青</button>
    <button type="button" class="bgBtn" id="bg-black">黒</button>
</div>


<div class="content">
  今回は「jquery-cookie」というライブラリを使って、文字サイズを変更できるボタン、背景色を変更できるボタンを実装していきます。今回は「jquery-cookie」というライブラリを使って、文字サイズを変更できるボタン、背景色を変更できるボタンを実装していきます。今回は「jquery-cookie」というライブラリを使って、文字サイズを変更できるボタン、背景色を変更できるボタンを実装していきます。今回は「jquery-cookie」というライブラリを使って、文字サイズを変更できるボタン、背景色を変更できるボタンを実装していきます。
</div>
!

CSS 

              
                :root {
  --pink-to-yellow: #ff82a5;
  --to-bgColor: white;
  --to-white: black;
}
html {
  font-size: 62.5%;
  transition: .2s;
}
body {
  font-size: 1.6rem;
  transition: .2s;
}
button:hover {
  cursor: pointer;
}

@media (max-width: 768px) {
    body {
        font-size: 1.4rem;
    }
}

.content {
  padding: 20px
}
.btn_wrap {
  margin: 20px;
}
.btn_wrap button {
  font-size: 1.6rem;
  padding: 5px 10px;
  border: 1px solid #d9d9d9;
  background: transparent;
  color: var(--to-white);
}
.btn_wrap button.is_active {
  background: var(--pink-to-yellow);
  color: var(--to-bgColor);
}
!

JS 

              
                $(function () {
  /* -------------------------
  // 背景色変更
  -------------------------- */
  let bg = $.cookie('bgColor');
  // cookieに値が保存されていた場合
  if (bg) {
    $('.bgBtn').removeClass('is_active');
    if (bg == 'bg-white') { 
      bgWhite();
    } else if (bg == 'bg-blue') { 
      bgBlue();
    } else if (bg == 'bg-black') { 
      bgBlack();
    }
  }

  $('.bgBtn').click(function () {
    $.cookie('bgColor', this.id, { expires: 7, path:'/' });
    $('.bgBtn').removeClass('is_active');
    if (this.id == 'bg-white') {
      bgWhite();
    } else if (this.id == 'bg-blue') { 
      bgBlue();
    } else if (this.id == 'bg-black') { 
      bgBlack();
    }
  });

  //背景色：白
  function bgWhite(){
    $('#bg-white').addClass('is_active');
    $('html').css('background', 'white'); //背景(白）
    $('html').css('color', 'black'); //文字色(黒）
    $(':root').css({
      "--to-white": 'black',
      "--pink-to-yellow": '#ff82a5',
      "--to-bgColor": 'white'
    });
  }
  //背景色：青
  function bgBlue(){
    $('#bg-blue').addClass('is_active');
    $('html').css('background', 'blue'); //背景(青）
    $('html').css('color', 'white'); //文字色(白）
    $(':root').css({
      "--to-white": 'white',
      "--pink-to-yellow": 'yellow',
      "--to-bgColor": 'blue'
    });
  }
  //背景色：黒
  function bgBlack(){
    $('#bg-black').addClass('is_active');
    $('html').css('background', 'black'); //背景(黒）
    $('html').css('color', 'white'); //文字色(白）
    $(':root').css({
      "--to-white": 'white',
      "--pink-to-yellow": 'yellow',
      "--to-bgColor": 'black'
    });
  }


  /* -------------------------
  // 文字サイズ変更
  -------------------------- */
  let size = $.cookie('fontSize');

  // cookieに値が保存されていた場合
  if (size) {
    $('.sizeBtn').removeClass('is_active');
    if (size == 'font-small') {
      fontSmall();
    } else if (size == 'font-middle') {
      fontMiddle();
    } else if (size == 'font-large') {
      fontLarge();
    }
  }
  
  $('.sizeBtn').click(function () {
    $.cookie('fontSize', this.id, { expires: 7, path:'/' });
    $('.sizeBtn').removeClass('is_active');
    if (this.id == 'font-small') {
      fontSmall();
    } else if (this.id == 'font-middle') {
      fontMiddle();
    } else if (this.id == 'font-large') {
      fontLarge();
    }
  });

  //文字サイズ：小
  function fontSmall(){
    $('#font-small').addClass('is_active');
    if(window.matchMedia("(max-width: 768px)").matches){
      $('body').css('font-size', '1.3rem');
    }else{ 
      $('body').css('font-size', '1.4rem');
    }
  }
  //文字サイズ：中
  function fontMiddle(){
    $('#font-middle').addClass('is_active');
    if(window.matchMedia("(max-width: 768px)").matches){
      $('body').css('font-size', '1.4rem');
    }else{ 
      $('body').css('font-size', '1.6rem');
    }
  }
  //文字サイズ：大
  function fontLarge(){
    $('#font-large').addClass('is_active');
    if(window.matchMedia("(max-width: 768px)").matches){
      $('body').css('font-size', '1.6rem');
    }else{ 
      $('body').css('font-size', '1.8rem');
    }
  }
});
!
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