var themeButton = $(".js-theme-switch");
themeButton.on({
click: function () {
var currentTheme = $("html").attr("data-theme");
var desiredTheme = currentTheme === "light" ? "dark" : "light";

localStorage.setItem('data-theme', desiredTheme);
$("html").attr("data-theme", desiredTheme);
$(".text", themeButton).html(currentTheme + " Mode");
}
});
var themeStatus = localStorage.getItem('data-theme'),
nextStatus = themeStatus === "light" ? "dark" : "light";
if (themeStatus !== null) {
$("html").attr("data-theme", themeStatus);
$(".text", themeButton).html(nextStatus + " Mode");
}

html{--color-bg: #FBFBFB;}
html&[data-theme="dark"]{--color-bg: #191923;}

<button class="js-theme-switch" type="button" aria-label="Theme Switch Button">
<span class="main-nav__item-icon">🌓</span><span class="text">Dark Mode</span>
</button>