实现原理

背景切换：一个白天的背景，再加一个黑夜的背景。黑色背景的堆叠顺序高于白色背景。当场景由白天转为黑夜时，黑色背景由透明逐渐转换为不透明，形成昼夜更替的效果。

日月变换：“太阳”和“月亮”其实是同一个元素，昼夜更替时将它所处的 div 盒子使用 transform 旋转 360°，看上去就是太阳落下然后月亮升起来了……

动画演示

代码示例

<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
    <meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1, user-scalable=0">
    <meta name="author" content="mengkun">
 
    <title>昼夜切换动画</title>
    <style>
    html, body {
        margin: 0;
        padding: 0;
        width: 100%;
        height: 100%;
    }
 
    /* 白天和黑夜的背景 */
    .mk-dark-mode-sky, 
    .mk-dark-mode-sky:before {
        content: "";
        position: fixed;
        left: 0;
        right: 0;
        top: 0;
        bottom: 0;
        z-index: 999999999;
        transition: 2s ease all;
    }
    .mk-dark-mode-sky {
        background: linear-gradient(#feb8b0, #fef9db);
    }
    .mk-dark-mode-sky:before {
        opacity: 0;
        background: linear-gradient(#4c3f6d, #6c62bb, #93b1ed);
    }
    .mk-dark-mode .mk-dark-mode-sky:before {
        opacity: 1;
    }
 
    /* 太阳和月亮 */
    .mk-dark-mode-planet {
        z-index: 1999999999;
        position: fixed;
        left: -50%;
        top: -50%;
        width: 200%;
        height: 200%;
        transform-origin: center bottom;
        transition: 2s cubic-bezier(.7, 0, 0, 1) all;
    }
    .mk-dark-mode-planet:after {
        position: absolute;
        left: 35%;
        top: 40%;
        width: 150px;
        height: 150px;
        border-radius: 50%;
        content: "";
        background: linear-gradient(#fefefe, #fffbe8);
    }
    </style>
    <script src="https://cdn.jsdelivr.net/npm/jquery@3.2.1/dist/jquery.min.js"></script>
</head>
<body>
    <div class="mk-dark-mode-sky">
        <div class="mk-dark-mode-planet"></div>
    </div>
    <script>
    /* 这里为了方便演示，点击页面中任意位置即可触发切换动画 */
    $("body").click(function() {
        $("body").toggleClass("mk-dark-mode");
        
        var angle = $('.mk-dark-mode-planet').data('angle') + 360 || 360;
        $('.mk-dark-mode-planet').css({'transform': 'rotate(' + angle + 'deg)' });
        $('.mk-dark-mode-planet').data('angle', angle);
    });
    </script>
</body>
</html>

参考资料

1.Good Morning, Goodnight.Nanou.https://codepen.io/jackiezen/pen/WMavEV
2.卡通日出日落CSS3动画特效.站长素材.http://demo.sc.chinaz.net/Files/DownLoad/webjs1/202004/jiaoben7518/