JSFiddle - React, Tailwind, and code Playground

HTML

<button id="changecss">Change CSS</button>

CSS

body {
    height: 1000px;
}

::-webkit-scrollbar {
    width:15px;
}
::-webkit-scrollbar-thumb {
    background-color:#999;
    border:solid #fff;
}
::-webkit-scrollbar-thumb:hover {
    background:#777;
}
::-webkit-scrollbar-thumb:vertical {
    border-width:6px 4px;
}

JavaScript

$("#changecss").on("click", function(){
   var ss = document.styleSheets[0];

    ss.insertRule('::-webkit-scrollbar {background-color: red}', 0);
});