---
date: '2024-02-17'
title: 'Adding a light-dark theme toggle'
description: "I dropped a light/dark theme toggle into the navigation of my site, replacing the prior reliance on the visitor's preference set at the OS level (though it does still consider this preference)."
tags: ['CSS', 'javascript', 'Eleventy', 'development']
---
I dropped a light/dark theme toggle into the navigation of my site, replacing the prior reliance on the visitor's preference set at the OS level (though it does still consider this preference).
I built the button as a short Liquid partial:
{% raw %}
```liquid
{% tablericon "placeholder" "Toggle theme placeholder" %}
{% tablericon "sun" "Toggle light theme" %}
{% tablericon "moon" "Toggle dark theme" %}
```
{% endraw %}
The `client-side` class above hides the button should the user have JavaScript disabled:
{% raw %]}
```html
```
{% endraw %}
And JavaScript is used to control the behavior of the toggle — first, in the `` of my base template:
{% raw %}
```liquid
{% capture js %}
{% render "../assets/scripts/theme.js" %}
{% endcapture %}
```
{% endraw %}
Where `theme.js` sets the initial state:
```javascript
;(async function() {
const prefersDarkScheme = window.matchMedia('(prefers-color-scheme: dark)').matches;
const currentTheme = localStorage?.getItem('theme');
if (currentTheme === 'dark') {
document.body.classList.add('theme__dark');
} else if (currentTheme === 'light') {
document.body.classList.add('theme__light');
} else if (prefersDarkScheme) {
document.body.classList.add('theme__dark');
} else if (!prefersDarkScheme) {
document.body.classList.add('theme__light');
}
})()
```
With the following JavaScript set in `assets/scripts/index.js` to add the `click` event listener for the theme toggle:
```javascript
;(async function() {
const btn = document.querySelector('.theme-toggle');
btn.addEventListener('click', () => {
document.body.classList.toggle('theme__light');
document.body.classList.toggle('theme__dark');
const prefersDarkScheme = window.matchMedia('(prefers-color-scheme: dark)').matches;
const currentTheme = localStorage?.getItem('theme');
let theme;
if (!currentTheme) localStorage?.setItem('theme', (prefersDarkScheme ? 'dark' : 'light'))
if (prefersDarkScheme) {
theme = document.body.classList.contains('theme__light') ? 'light' : 'dark';
} else {
theme = document.body.classList.contains('theme__dark') ? 'dark' : 'light';
}
localStorage?.setItem('theme', theme);
});
})()
```
Finally, the theme is updated based on the body class applied by the JavaScript, updating the variable values that define my site's theme:
```css
/* theme toggle */
.theme-toggle,
.theme-toggle svg {
cursor: pointer;
}
.theme-toggle > .light svg { stroke: var(--sun) !important; }
.theme-toggle > .dark svg { stroke: var(--moon) !important; }
.theme-toggle > .light ,
.theme-toggle > .dark {
display: none;
}
:is(.theme__light, .theme__dark) .theme-toggle > .placeholder {
display: none;
}
.theme__dark .theme-toggle > .light {
display: inline;
}
.theme__dark .theme-toggle > .dark {
display: none;
}
.theme__light .theme-toggle > .light {
display: none;
}
.theme__light .theme-toggle > .dark {
display: inline;
}
...
:root body.theme__light {
--text-color: var(--color-darkest);
--background-color: var(--color-lightest);
--text-color-inverted: var(--color-lightest);
--background-color-inverted: var(--color-darkest);
--accent-color: var(--blue-600);
--accent-color-hover: var(--blue-800);
--selection-color: var(--accent-color);
--gray-light: var(--gray-200);
--gray-lighter: var(--gray-50);
--gray-dark: var(--gray-700);
--brand-github: #333;
}
:root body.theme__dark {
--text-color: var(--color-lightest);
--background-color: var(--color-darkest);
--text-color-inverted: var(--color-darkest);
--background-color-inverted: var(--color-lightest);
--accent-color: var(--blue-400);
--accent-color-hover: var(--blue-200);
--gray-light: var(--gray-900);
--gray-lighter: var(--gray-950);
--gray-dark: var(--gray-300);
--brand-github: #f5f5f5;
}
```
With those changes in place, visitors can toggle whichever theme they'd prefer and their preference is persisted in `localStorage` should it be available.
**EDIT:** Many thanks to [Tixie Salander](https://mastodon.guerilla.studio/@tixie) for [guiding me to a solution that improves the initial load experience](https://mastodon.guerilla.studio/@tixie/111950371813634672).