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HTML

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Insecure Resource

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CSS

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JavaScript

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Babel includes JSX processing.

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HTML 

              
                <body id="mainBody">
	<div class="btn-group">
		<a target="_blank" href="https://www.insidethediv.com/how-to-make-website-dark-mode-css-html-javascript-example">More</a>
		
		<button onclick="MakeSiteDark()" id="btnMakeDark">Dark</button>
		<button onclick="MakeSiteWhite()" id="btnMakeWrite">White</button>
		
	</div>
	<div class="container">
		<h1>How to make website dark mode CSS HTML JavaScript Code Pen</h1>
		Lorem ipsum dolor sit amet consectetur adipisicing elit. Illo praesentium eius quaerat molestiae eveniet, rem facilis explicabo iusto aliquid amet repellendus recusandae accusantium cumque expedita eligendi, repudiandae commodi. Laudantium molestiae eveniet veritatis provident possimus enim. Tempora, voluptatum. Nobis doloribus blanditiis beatae, odio libero, fuga! Eaque aspernatur, atque quasi, vel sapiente natus fuga. Quidem eaque pariatur earum quae fuga consectetur excepturi eligendi recusandae debitis ipsam dolore, optio est voluptates blanditiis asperiores officia eum rem ipsa possimus reiciendis. Dolorem impedit incidunt, nesciunt ea dignissimos molestiae distinctio praesentium architecto rem quidem culpa ab? Officiis, est? Maxime itaque iusto veritatis maiores. Officiis eaque, ex!
	</div>
</body>
!

CSS 

              
                .btn-group{
	position: fixed;
	top: 10px;
	right: 10px;
}
.btn-group button{
	font-size: 17px;
	padding: 2px 20px;
	cursor: pointer;
	border: 0px;
	box-shadow: 0px 0px 1px 2px #eee;
	cursor: pointer;
}

.btn-group #btnMakeDark{
	background: #333;
	color: #fff;
}

.btn-group #btnMakeWrite{
	background: #fff;
	text-color: #333;
}

.container{
	margin-top: 50px;
}

.dark-body{
	background: #000;
	color: #fff;
}

.white-body{
	background: #fff;
	color: #000;
}
!

JS 

              
                function MakeSiteDark(){
	var mainBody = document.getElementById('mainBody');
	mainBody.classList.remove('white-body');
	mainBody.classList.add('dark-body');
}

function MakeSiteWhite(){
	var mainBody = document.getElementById('mainBody');
	mainBody.classList.remove('dark-body');
	mainBody.classList.add('white-body');
}
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999px

Console