SOURCE

function change() {
    const color = document.getElementById('select').value
    document.getElementById('div').className = color
}

<div class="red" id="div">
	<div class="item">
		111
	</div>
</div>
<select id="select">
    <option value="blue">blue</option>
    <option value="pink">pink</option>
    <option value="orange">orange</option>
</select>
<button onclick="change()">换肤</button>

.item {
    color: #fff;
}

.red .item {
    background: red;
}

.blue .item {
    background: blue;
}

.pink .item {
    background: pink;
}

.orange .item {
    background: orange;
}