You can see an example below, basically forget adding margin or padding there, just increase the width/height of scroll area, and decrease the width height of thumb/track.
Quoted from how to customise custom scroll?
body {
min-height: 1000px;
font-family: sans-serif;
}
div#container {
height: 200px;
width: 300px;
overflow: scroll;
border-radius: 5px;
margin: 10px;
border: 1px solid #AAAAAA;
}
div#content {
height: 1000px;
outline: none;
padding: 10px;
}
::-webkit-scrollbar {
width: 14px;
}
::-webkit-scrollbar-thumb {
border: 4px solid rgba(0, 0, 0, 0);
background-clip: padding-box;
border-radius: 9999px;
background-color: #AAAAAA;
}
<div id="container">
<div id="content" contenteditable>
Click to type...
</div>
</div>