css 实现黑夜白天主题

<style>
[data-theme=light] .main{
  background:red;
}
[data-theme=dark] .main{
  background:blue;
}
</style>

<div data-theme="light"><span class="main">light</span></div>
<div data-theme="dark"><span class="main">dark</span></div>

效果如下: