Dark Theme Toggle - How to create a dark mode switch in a few lines of JS + CSS

Switch to dark theme in a couple of lines of Javascript + CSS 🤯

The purpose of this project is to create a toggle button to switch the theme of your website in a few lines of css and javascript code.

Basically we have this file structure:

HTML

In the html file we create a button that calls on click a function toggleTheme()

<button onclick="toggleTheme()">CHANGE ME</button>

Javascript

In javascript instead we manage the switch, first of all let's set the body and the default mode:

const body = document.querySelector ('body')
const defaultTheme = 'light'

This will be used to define the mode chosen by us the first time the user arrives on our page.
Moving on, let's create the function toggleTheme()

const toggleTheme = () => {
  let activeTheme = localStorage.getItem('theme')
  activeTheme == 'light' ? setTheme('dark') : setTheme('light')
}

this is the core of our project, as you can see there is an activeTheme control variable that checks if a preference has been saved in the user's localStorage, on the basis of the value that our localStorage will provide us, we set the opposite value.

So if localStorage.getItem('theme') has the value 'light' we will set 'dark' as parameter for setTheme() function to change the mode.

So what does setTheme() do?

const setTheme = (theme) => {
  localStorage.setItem('theme', theme)
  body.setAttribute('data-theme', theme)
}

In this function we do only two things:

1. save the new selected mode in our user's localStorage.
2. add to our body tag the data-theme that will manage the correct css

and to complete our plugin we launch the function that takes care of setting the default mode at startup:

const checkDefaultTheme = () => {
  let savedTheme = localStorage.getItem('theme')
  savedTheme ? body.setAttribute('data-theme', savedTheme) : setTheme(defaultTheme)
}

checkDefaultTheme();

this function starts as soon as the user arrives on the page, then we check if he is a returning user and therefore has already chosen his preferred mode, if it is not present in the local storage we proceed with setting the default mode.

CSS

Let's move to the CSS
we need very few lines of css because we will use variables.

:root {
  --main-bg: white;
  --main-txt: black;
  --button-bg: black;
  --button-txt: white;
}
[data-theme="dark"] {
  --main-bg: black;
  --main-txt: white;
  --button-bg: white;
  --button-txt: black;
}
body {
  background-color: var(--main-bg);
  color: var(--main-txt);
}

in the root we define the colors that we will use for the default mode (in our case the light) then we define the dark mode with [data-theme = "dark"] in which we will change the colors to the same root variables.

And so the magic begins... our body will work on the colors set in the variables and depending on the [data-theme] it will invert them

❤️❤️❤️ feel free to use this snippet, you can find the entire project here on github aaaand, if you liked it, star it on github