纯css随鼠标移动渐变色层

<div class="art">
<div class="box lvl6">
<div class="box lvl5">
<div class="box lvl4">
<div class="box lvl3">
<div class="box lvl3">
<div class="box lvl3">
<div class="box lvl2">
<div class="box lvl2">
<div class="box lvl2">
<div class="box lvl2">
<div class="box lvl1">
<div class="box lvl1">
<div class="box lvl1">
<div class="box lvl1">
<div class="box lvl1">
<div class="box lvl1">
<div class="box lvl1">  
<div class="box lvl1">
<div class="box lvl1">
<div class="box lvl1">
<div class="box lvl1">
<div class="box lvl1">
<div class="box lvl1">
<div class="box lvl1">
<div class="box lvl1">
<div class="box lvl1">
<div class="box lvl1">
<div class="box lvl1">
<div class="box lvl1">
<div class="box lvl1">
<div class="box lvl1">
<div class="box lvl1">
<div class="box lvl1">
<div class="box lvl1">
<div class="box lvl1">  
</div>
</div>
</div>
</div>
</div>
</div>
</div>