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HTML

              
                <details id=details-1> <summary>①デフォルト</summary>
<li>折りたたむ内容</li>
<li>折りたたむ内容</li>
<li>折りたたむ内容</li>
</details>

<details id=details-2> <summary>②矢印を消去</summary>
<li>折りたたむ内容</li>
<li>折りたたむ内容</li>
<li>折りたたむ内容</li>
</details>

<details id=details-3> <summary>③矢印を■に変更</summary>
<li>折りたたむ内容</li>
<li>折りたたむ内容</li>
<li>折りたたむ内容</li>
</details>

<details id=details-4> <summary>④アコーディオンメニューの全体の背景色を変更</summary>
<li>折りたたむ内容</li>
<li>折りたたむ内容</li>
<li>折りたたむ内容</li>
</details>

<details id=details-5> <summary>⑤アコーディオンメニューのsummaryの背景色のみ変更</summary>
<li>折りたたむ内容</li>
<li>折りたたむ内容</li>
<li>折りたたむ内容</li>
</details>
              
            
!

CSS

              
                /*①*/
#details-1 {

}
/*②*/
#details-2 summary::-webkit-details-marker {
  display: none;
}
/*③*/
#details-3 summary::-webkit-details-marker {
  display: none;
}

#details-3 summary:before {
  content: '■ ';
}
/*④*/
#details-4 {
background-color:#DDFFFF;
width:400px;
}
/*⑤*/
#details-5 summary{
background-color:#FFFF99;
width:450px;
}
              
            
!

JS

              
                
              
            
!
999px

Console