前言

从来没学过前端，以后也不打算学，目前的前端生态已经相当成熟了，写个网页和word技术含量差不多，毕竟网页也只是描述语言而已。

巧用F12

会用F12查看你要修改的元素的id和class非常重要
F12也能查看源码

创建JS文件定位id,判断class

hexo 随机 indeximg,整体.js文件的代码:

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var bannerimg =[
  "url(https://ae01.alicdn.com/kf/H21b5f6b8496141a1979a33666e1074d9x.jpg)",
  "url(https://picsum.photos/id/767/3069/2048)",
  "url(https://picsum.photos/id/800/4663/3109)",
  "url(https://picsum.photos/id/459/2310/1534)",
  "url(https://picsum.photos/id/458/5184/3456)",
  "url(https://picsum.photos/id/317/1935/1089)"
];
//获取banner图片总数，生成随机数
var bannerindex =Math.floor(Math.random() * bannerimg.length);
var bgid = document.getElementById("page-header");
if(bgid.className =="full_page")                                //精确到class重要插桩
{document.getElementById("page-header").style.backgroundImage = bannerimg[bannerindex];}

定位ID
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var bgid = document.getElementById("page-header");

判断class类并修改(赋值)
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if(bgid.className =="full_page")                                //精确到class重要插桩
  {document.getElementById("page-header").style.backgroundImage = bannerimg[bannerindex];}