MediaWiki:Gadget-toggleDarkMode.js

 ;(function($, mw) {
	 var DARKMODE_COOKIE = "darkmode",
	 	 currentDark = localStorage.getItem(DARKMODE_COOKIE) == 'true',
		 portletLink,
		 formMade;

	 var self = {
		 init: function() {
			portletLink = mw.util.addPortletLink(
				'p-personal',
				'',
				'',
				'pt-skin-toggles',
				'Toggle darkmode',
				null,
				$('#pt-userpage, #pt-anonuserpage')
			);

			$(portletLink).find('a').addClass('oo-ui-icon-advanced').click(function(e) {
				e.preventDefault();

				if (currentDark) {
					currentDark = false;
				}

				localStorage.setItem(DARKMODE_COOKIE, currentDark);

				if (currentDark) {
					$('body').addClass('wgl-darkmode')
					$('body').removeClass('wgl-lightmode')
				} else {
					$('body').addClass('wgl-lightmode')
					$('body').removeClass('wgl-darkmode')
				}

		 	})
		}
	}

	mw.loader.using(['mediawiki.util'], function () {
		$(self.init);
	})

 }(jQuery, mediaWiki));