Gadget-DarkToggle.js

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// Toggle a dark theme for supported skins
// Add to Minerva footer list

function setCookie(c_name, value, expiredays) {
	var exdate = new Date();
	exdate.setDate(exdate.getDate() + expiredays);
	document.cookie = c_name + '=' + escape(value) + ';path=/' + ((expiredays === null) ? '' : ';expires=' + exdate.toGMTString());
}
function getCookie(c_name) {
	if (document.cookie.length > 0) {
		c_start = document.cookie.indexOf(c_name + '=');
		if (c_start != -1) {
			c_start = c_start + c_name.length + 1;
			c_end = document.cookie.indexOf(';', c_start);
			if (c_end == -1) c_end = document.cookie.length;
			return unescape(document.cookie.substring(c_start, c_end));
		}
	}
	return '';
}

$(function() {
	if (window.matchMedia('(prefers-color-scheme: dark)').matches) {
		$('body').addClass('dark');
		$('body').removeClass('light');
	}
	window.matchMedia('(prefers-color-scheme: dark)').addListener(function(e) {
		if (e.matches) {
			$('body').addClass('dark');
			$('body').removeClass('light');
		} else {
			$('body').addClass('light');
			$('body').removeClass('dark');
		}
	});

	var isDark = false;
	if (getCookie('darkTheme') == 'on')
		isDark = true;
	if (mw.config.get('skin') == 'nimbus')
		$('#wiki-login').prepend('<div id="toggleContainer" style="float:left;position:relative;right:5px;cursor:pointer"><img id="themeToggle" src="https://wiki.gallery/images/dark/moon/' + (isDark ? 'yellow' : 'white') + '.png" title="Toggle night theme"/></div>');
	else
		$('#footer-places').append('<li id="toggleContainer" style="display:inline-block;cursor:pointer"><a id="themeToggle" href="javascript:;">Toggle night theme</a></li>');
	$('#themeToggle').click(function() {
		if (isDark)
			setCookie('darkTheme', '', -1);
		else
			setCookie('darkTheme', 'on', 999);
		location.reload();
	});
});

MediaWiki:Gadget-DarkToggle.js