如何通过js代码或者css样式控制滚动条滑块？

能否通过js代码动态的控制页面滚动条滑块的颜色显示与隐藏？例如控制（::-webkit-scrollbar-thumb）逐渐透明直至消失

效果图

页面构成

 <span class="box" id="box-lj-sgj">
                <i class="hd"></i>
                <i class="hdt"></i>
 </span>

css

.tab-con .box,.tab-con .box .hd,.tab-con .box .hdt{
        display: inline-block;
        *display: inline;
        zoom:1;
}
.tab-con .box {
        cursor: pointer;
        width: 195px;
        height: 3px;
        background-color: #bdbdbd;
        position: relative;
        vertical-align: middle;
    }
.tab-con .box .hd {
        width: 17px;
        height: 18px;
        background-color: #56a3fc;
        border-radius: 6px;
        position: absolute;
        left: 0;
        top: -8px;
        cursor: pointer;
        margin: 0;
}
.tab-con .box .hdt {
        position: absolute;
        top: 0;
        left: 0;
        height: 3px;
        width: 0;
        background-color: #56a3fc;
        margin: 0;
}

js代码

function slider(obj,num){
    var box = document.getElementById(obj);
         var hd = box.children[0];
         var hdt = box.children[1];
         //鼠标按下事件  只要鼠标按下时是在hd上触发就可以
         hd.onmousedown = function(event){
             var event = event || window.event;
             //event.clientX 鼠标按下时的x坐标
             //this.offsetLeft 圆点到box的距离
             //leftval box距离左边边界的距离
             var leftval = event.clientX - this.offsetLeft;
             var _this = this;
             //滑动事件 作用在document上，因为如果是作用在hd.onmousemove，滑动时可能会出现鼠标脱离hd范围（范围太小）而停止滑动，但是此时鼠标按下还没有弹起还处于滑动状态，所以就产生了bug
             document.onmousemove = function(event){
                 var event = event || window.event;
                 //hd.style.left = _this.offsetLeft + "px";
                 //offsetLeft 是要有left值的前提下才产生，不能offsetLeft先赋值给left
                 hd.style.left = event.clientX - leftval + "px";

                 var far = parseInt(hd.style.left);
                   
                 //临界值判断
                 if(far < 0){
                     hd.style.left = 0;
                 }
                 else if(far > 195){
                     hd.style.left = 195+"px";
                 }
                 
                 hdt.style.width = hd.style.left;
                 //保留两位小数
//               var result = Math.floor((parseInt(hdt.style.width)/195 * num) * 100) / 100;
               //取整
                 var result = parseInt(parseInt(hdt.style.width)/195 * num);
                 //效果图上的476
                 box.nextSibling.nextSibling.value = result;
//               console.log(box.nextSibling.nextSibling);
                 window.getSelection ? window.getSelection().removeAllRanges() : document.selection.empty();//清除选中的文本
             }
        //当鼠标弹起时清除滑动时运行的函数，因为滑动结束时鼠标可能不在hd上，所以用document监听弹起事件
             document.onmouseup = function(){
                 document.onmousemove = null;
             }

         }
         //点击选择位置
         box.onclick = function(event){
             var event = event || window.event;
             var boxWidth = event.clientX-this.offsetLeft;
             if(boxWidth > 195){
                     boxWidth = 195;
             }
             hdt.style.width = hd.style.left = boxWidth + 'px';
             var result = parseInt(parseInt(hdt.style.width)/195 * num);
             box.nextSibling.nextSibling.value = result;
             console.log(event.clientX-this.offsetLeft);
         }
}

调用slider(obj,num)函数即可