JSFiddle - React, Tailwind, and code Playground

HTML

<div class="main">
<section class="blockInMain">
<div class="link6"><span>День</span> <span class="active">Ночь</span></div>
</section>
</div>

CSS

.link6 span.active
{
	color: #1870a6;
	border-bottom: dashed 1px;
	cursor: pointer;
}

JavaScript

$(function(){
	$('.blockInMain .link6 .active').click(function(){
		$('.blockInMain .link6 span').toggleClass('active');
		$('.main').toggleClass('night');
	});
});