通过点击主题按钮修改颜色
实现效果：

html代码：

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>点击元素按钮切换主题</title>
    <link rel="stylesheet" href="./index.css">
</head>
<body>
    <ul>
        <li class="theme-1">主题1</li>
        <li class="theme-2">主题2</li>
        <li class="theme-3">主题3</li>
    </ul>

    <div class="article theme-1" id="article">
        <div class="header">这是一首诗</div>
        <div class="main">
            <div class="art-title">锄禾</div>
            <div class="art-content">
                锄禾日当午，<br>
                汗滴禾下土。<br>
                谁知盘中餐，<br>
                粒粒皆辛苦。<br>
            </div>
        </div>
        <div class="footer">作者：李绅</div>
    </div>
    <script src="./index.js"></script>
</body>
</html>

css代码：

* {
    margin: 0;
    padding: 0;
    list-style: none;
}

ul {
    display: flow-root;
    margin-left: 100px;
    cursor: pointer;
    margin-top: 30px;
}

li {
    width: 80px;
    height: 40px;
    line-height: 40px;
    text-align: center;
    margin-right: 50px;
    color: #fff;
    float: left;
}

li.theme-1 {
    background-color: red;
}

li.theme-2 {
    background-color: blue;
}

li.theme-3 {
    background-color: green;
}

.article {
    width: 240px;
    margin: 30px 0 0 150px;
    text-align: center;
}

.header {
    height: 60px;
    line-height: 60px;
    font-size: 24px;
    color: #fff;
    font-weight: 500;
    padding-left: 10px;
}

.main {
    padding: 20px;
    width: 200px;
}

.art-title {
    font-size: 22px;
    font-weight: 700;
    height: 60px;
    line-height: 60px;
}

.footer {
    height: 40px;
    line-height: 40px;
    color: black;
}

/* 抽取公共样式 */

/* .theme-1 */
.theme-1 .header {
    background-color: red;
}

.theme-1 .main {
    background-color: #ecf5ff;;
}

.theme-1 .art-content {
    color: red;
}

.theme-1 .footer {
    background-color: red;
}

/* .theme-2 */
.theme-2 .header {
    background-color: blue;
}

.theme-2 .main {
    background-color: #e1f3d8;;
}

.theme-2 .art-content {
    color: blue;
}

.theme-2 .footer {
    background-color: blue;
}

/* .theme-3 */
.theme-3 .header {
    background-color: green;
}

.theme-3 .main {
    background-color: #fde2e2;;
}

.theme-3 .art-content {
    color: green;
}

.theme-3 .footer {
    background-color: green;
}

JS代码：

var lis = document.getElementsByTagName('li');
var article = document.getElementById('article');


lis[0].onclick = function() {
    article.className = 'article theme-1';
}

lis[1].onclick = function() {
    article.className = 'article theme-2';
}

lis[2].onclick = function() {
    article.className = 'article theme-3';
}