Create a Theme Switch


This is a simple tutorial on how to make a simple theme switcher.

First step is to create the themes you'll want on your site. You may just want a light and dark mode, however, you may also want other themes. As an example: a clown theme that makes the page absurdly colorful, like a GeoCities site in the early 2000s. For these themes, you can use CSS variables set on the html parent tag(all other elements are considered children of html). Variables can be set in CSS using double hyphens followed by a sequence of text that can use numbers(but not at the start), letters(capital or lowercase), underscores, and hyphens; these variables can then be accessed within your text using the var() function with the name of the variable(including the starting hyphens). You can change out the CSS variables using different selectors on the html tag within the CSS, for me: I'm using the [data-theme] attribute; however, you can use classes if you want. Here is the CSS I wrote:

html[data-theme=light] {
    background: #fff;
    color: #000;
    --buttonBackground: #fefefe;
    --buttonBorder: #ccc;
    --buttonColor: #001;
html[data-theme=dark] {
    background: #000;
    color: #fff;
    --buttonBackground: #101010;
    --buttonBorder: #333;
    --buttonColor: #fff;
html[data-theme=clown] {
    background: #f00;
    color: #00f;
    --buttonBackground: #050;
    --buttonBorder: #0a0;
    --buttonColor: #0f0;
button {
    background: var(--buttonBackground);
    border: var(--buttonBorder) solid 3px;
    color: var(--buttonColor);

Now we need to create a button that we will use to change the [data-theme] attribute. You (probably) already know what buttons are, so I won't go in depth about then. However: you will need to add an [id] to the button, this will come in play shortly.

<button id="theme-switch">Switch Theme</button>

Now, it is time to write the JavaScript. First, you'll want to make a constant with the themes you filled into your CSS, for me, that was dark, light, and clown. I'll name this themes.

const themes = ["light", "dark", "clown"];

We now need to query the DOM for our theme switch button. This can be done in 2 ways: document.querySelector or document.getElementById; personally, I prefer querySelector, as it allows you to write a CSS selector to get an element from the DOM, allowing for shorter, more digestible, code.

We have to add a click event to the button. This can be done in 2 ways within JavaScript: addEventListener("click", ...) and onclick. I personally like addEventListener more, so I'll use that. Within the addEventListener function, you need to put a function. This function will control the logic of our theme switch. I'll call this function SwitchTheme, to reflect it's functionality.

const ThemeSwitchButton = document.querySelector("#theme-switch");
ThemeSwitchButton.addEventListener("click", SwitchTheme());
function SwitchTheme() {}

Explaining each part of this function as it's written out would take awhile, so instead, I'll add comments to the ends of each line giving a short explanation. Also, for the sake of shortness, I've placed document.documentElement inside the constant HTML, giving us access to the root element in the DOM(html).

let CurrentTheme = themes.indexOf(HTML.dataset.theme); // Gets how far in the current theme is into the "themes" constant.
if (CurrentTheme + 1 >= themes.length) { // Checks if it's at the end of the array,
    HTML.dataset.theme = themes[0]; // If so, reset at the start.
    return; // Ends the function here, preventing next bit of code from running.
HTML.dataset.theme = themes[CurrentTheme + 1]; // Goes to the next theme.

Now we have a finished product. Here's the expected output:

See Code