var $affectedElements = $("li, p, h1, h2, h3, h4, h5, h6, label"); // Can be extended, ex. $("div, p, span.someClass")

// Storing the original size in a data attribute so size can be reset
$affectedElements.each( function(){
var $this = $(this);
$this.data("orig-size", $this.css("font-size") );
});

$("#btn-increase").click(function(){
    changeFontSize(1);
})

$("#btn-decrease").click(function(){
    changeFontSize(-1);
})

$("#btn-orig").click(function(){
$affectedElements.each( function(){
        var $this = $(this);
        $this.css( "font-size" , $this.data("orig-size") );
    });
})

function changeFontSize(direction){
    $affectedElements.each( function(){
        var $this = $(this);
        $this.css( "font-size" , parseInt($this.css("font-size"))+direction );
    });
}  

const checkbox = document.getElementById("checkbox");
const isDarkMode = localStorage.getItem("darkMode") === "true";
checkbox.checked = isDarkMode;
const toggleDarkMode = () => {
    const isDarkMode = checkbox.checked;
    document.body.classList.toggle("dark", isDarkMode);
    localStorage.setItem("darkMode", isDarkMode);
};
checkbox.addEventListener("change", toggleDarkMode);
toggleDarkMode();