使用方法：
1、head引入css文件
<link rel="stylesheet" href="css/style.css" />
2、body引入部分
<div class="wrapper"> <div class="star" id="star1">.</div> <div class="star" id="star2">.</div> <div class="star" id="star3">.</div> <div class="star" id="star4">.</div> <div class="star" id="star5">.</div> <div class="moon"></div> <div class="mountain"></div> <div class="tree l1" id="tree1"></div> <div class="tree l1" id="tree2"></div> <div class="tree l1" id="tree3"></div> <div class="tree l1" id="tree4"></div> <div class="tree l2" id="tree5"></div> <div class="tree l2" id="tree6"></div> <div class="tree l2" id="tree7"></div> <div class="tree l2" id="tree8"></div> <div class="tree l3" id="tree9"></div> <div class="tree l3" id="tree10"></div> <div class="tree l3" id="tree11"></div> <div class="tree l3" id="tree12"></div> </div>
