JavaScript 変更 CSS スタイルのサンプル コード 4 つをお勧めします

.style1{margin:10px auto ;background-color:#9999FF; display:block;color:White; border:1px solid white; padding:10px 25px; font-size:18px; }

.style1:hover{ background-color:#66B3FF; cursor:pointer;}

.style2{margin:10px auto ;background-color:gray; display:block;color:black; border:1px solid white; padding:10px 25px; font-size:18px; }

.style2:hover{ background-color:black; color:White; cursor:pointer}

 <p>
  <input id="btnB" type="button" name="btnLogin" value="登录" class="style1" />
  <p id="tool">
   <input type="button" value="【obj.style.className】更改样式" onclick="changeBackgroundColor()"/>
   <input type="button" value="【obj.style.cssText】更改样式" onclick="changeFontSize()" />
   <input type="button" value="【obj.className】更改样式" onclick="addRadius()" />
   <input type="button" value="更改外联css样式" onclick="recover()" />
  </p>
 </p>

 function changeStyle1() {
   var obj = document.getElementById("btnB");
   obj.style.backgroundColor= "black";

 }

 function changeStyle2() {
   var obj = document.getElementById("btnB");
   obj.style.cssText = "background-color:black; display:block;color:White;

}

 function changeStyle3() {
   var obj = document.getElementById("btnB");
   //obj.className = "style2";
   obj.setAttribute("class", "style2");
 }

<link href="css1.css" rel="stylesheet" type="text/css" id="css"/>

function changeStyle4() {
   var obj = document.getElementById("css");
   obj.setAttribute("href","css2.css");
 }