css如何自定义滚动条（代码）

<p id="inp" class="test" contenteditable="true" ></p>

.test {
 display: inline-block;
 margin: 60px 40%;
 width: 280px;
 padding: 5px 4px;
 min-height: 20px;
 line-height: 20px;
 max-height: 72px;
 border: 1px solid #ccc;
 font-size: 12px;
 word-wrap: break-word;
 overflow-x: hidden;
 overflow-y: auto;
 -webkit-user-modify: read-write-plaintext-only;
 border-radius: 4px;
 }
 .test::-webkit-scrollbar {
 width: 4px;
 height: 1px;
 }
 .test::-webkit-scrollbar-thumb {
 border-radius: 4px;
 -webkit-box-shadow: inset 0 0 5px rgba(0,0,0,0.2);
 background: #C8C8C8;
 }
 .test::-webkit-scrollbar-track {
 border-radius: 4px;
 background-color: #FFFFFF;
 }