CSS Theme Selector with Automatic Mode [Tutorial]

This tutorial shows you how to create a theme selector in Svelte, enabling multiple theme options for your website. It also includes an automatic theme that adapts to the user's device settings.

Although it's implemented in Svelte, most of the functionality relies on standard HTML and CSS, making it easy to replicate in other frameworks.

The source code for the project can be found here: https://github.com/ivarnm/theme_selector

Live demo: https://theme-selector-inm.vercel.app/

Setting up the project:

If you don't already have an existing Svelte project, you can create one by following Svelte's getting started guide: https://svelte.dev/docs/introduction

Add color variables:

We will use CSS variables to define our color themes. First, create a src/styles/global.css file and add the following CSS:

:root {
  --neutral-0: white;
  --neutral-10: #fffcf1;
  --neutral-30: #d2d2d2;
  --neutral-40: #b7b7b7;
  --neutral-60: #666666;
  --neutral-70: #333333;
  --neutral-100: black;

  --primary-60: #696d90;
  --primary-70: #3D405B;
  --primary-80: #303349;

  --secondary-70: #5a7b6b;
  --secondary-80: #456153;
}

.dark-theme {
  --neutral-0: black;
  --neutral-10: #1a1a1a;
  --neutral-30: #3d3d3d;
  --neutral-40: #595959;
  --neutral-60: #999999;
  --neutral-70: #cccccc;
  --neutral-100: white;

  --primary-60: #979ec7;
  --primary-70: #a7aed6;
  --primary-80: #b8bfe9;

  --secondary-70: #79BEA5;
  --secondary-80: #89cfb5;
}

.warm-theme {
  --neutral-0: #fff7e0;
  --neutral-10: #ffedcc;
  --neutral-30: #ffdbb7;
  --neutral-40: #ffb89d;
  --neutral-60: #ff9473;
  --neutral-70: #ff5733;
  --neutral-100: #4d2600;

  --primary-60: #f28e2b;
  --primary-70: #d65a31;
  --primary-80: #c44536;

  --secondary-70: #e59572;
  --secondary-80: #cf6448;
}

html {
  font-family: 'Inter', sans-serif;
  background-color: var(--neutral-10);
}

body {
  margin: 0 auto;
  max-width: 870px;
  color: var(--neutral-70);
}

This defines a color palette for the light theme on the root element and adds a dark and warm theme. Which colors and themes you want is, of course, up to you. These are just some examples. You also see how we can use these variables to set the page's background color and the body's color property.

Next, add a src/routes/ layout.svelte file and add the following code to import the CSS file globally:

<script>
  import "../styles/global.css";
</script>

Create a dropdown menu:

I prefer to have my theme selector inside a dropdown menu, so I'll make a reusable DropdownMenu component that we can use. If you want to do the same, you can create a src/lib/components/DropdownMenu.svelte file and add the following:

<script lang="ts">
  let menuOpen = false;

  const toggleMenu = () => {
    menuOpen = !menuOpen;
  };

  const handleDropdownFocusLoss = (event: FocusEvent) => {
    const focusedElement = event.relatedTarget instanceof HTMLElement ? event.relatedTarget : null;
    const menuElement = event.currentTarget instanceof HTMLElement ? event.currentTarget : null;

    // Check if the new focus is inside the menu
    if (focusedElement && menuElement && menuElement.contains(focusedElement)) {
        return;
    }

    menuOpen = false;
  };
</script>

<div class="container">
  <div class="menu" on:focusout={handleDropdownFocusLoss}>
    <button class="icon-btn" on:click={toggleMenu}>
      <slot name="icon"></slot>
    </button>

    {#if menuOpen}
      <div class="dropdown">
        <slot name="dropdown"></slot>
      </div>
    {/if}
  </div>
</div>

<style>
  .menu {
    position: relative;
  }

  .icon-btn {
    background-color: transparent;
    border: none;
    cursor: pointer;
    padding: 6px;
    margin: 0;
    border-radius: 50%;
    overflow: hidden;
    width: 49px;
    height: 49px;
    display: flex;
    align-items: center;
    justify-content: center;
  }

  .icon-btn:hover {
    outline: none;
    background-color: var(--neutral-30);
  }

  .dropdown {
    position: absolute;
    top: 60px;
    right: 0;
    background-color: var(--neutral-30);
    border: 1px solid var(--neutral-40);
    color: var(--neutral-100);
    padding: 5px;
    z-index: 100;
    box-shadow: 0 0 10px rgba(0, 0, 0, 0.1);
    display: flex;
    flex-direction: column;
    gap: 10px;
    width: 200px;
  }

  .dropdown :global(button) {
    padding: 10px 15px;
    font-size: 20px;
    display: flex;
    align-items: center;
    gap: 10px;
    width: 100%;
  }

  .dropdown :global(button:hover) {
    background-color: var(--neutral-40);
  }
</style>

Create the theme selector component:

Now it's time to create the component that will handle the theme selection. Create the file
src/lib/components/ThemeSelector.svelte and add the following:

<script lang="ts">
  import { onMount } from 'svelte';
  import { writable } from 'svelte/store';
  import DropdownMenu from '$lib/components/DropdownMenu.svelte';

  // If you want to use the theme variable in other components, you can move it to a dedicated ts/js file and import it here instead
  let theme = writable<string>("system");

  // Define your themes and their names.
  const THEMES = [
      { value: 'system', label: 'Automatic' },
      { value: 'light', label: 'Light' },
      { value: 'dark', label: 'Dark' },
      { value: 'warm', label: 'Warm' },
  ];

  onMount(() => {
    // Prevents the code from running on the server
    if (typeof window == 'undefined') return;

    let storedTheme = localStorage.getItem('theme');

    // Get the user's system theme preference
    let systemTheme = window.matchMedia('(prefers-color-scheme: dark)').matches ? 'dark' : 'light'; 

    if (storedTheme && THEMES.some(themeOption => themeOption.value === storedTheme)) {
      theme.set(storedTheme);
    } 
    else {
      theme.set('system');
      applyTheme(systemTheme);
    }

    // Update the automatic theme when the system theme changes if the theme is set to automatic
    window.matchMedia('(prefers-color-scheme: dark)').addEventListener('change', e => {
      systemTheme = e.matches ? 'dark' : 'light';
      if (storedTheme === 'system') {
        applyTheme(systemTheme);
      }
    });

    theme.subscribe(value => {
      if (value === 'system') {
        applyTheme(systemTheme);
      } 
      else {
        applyTheme(value);
      }

      localStorage.setItem('theme', value);
      storedTheme = value;
    });
  });

  function applyTheme(theme: string) {
    document.documentElement.className = "";
    if(theme === 'system') return;

    // Add the theme class (e.g dark-theme) to the document element to apply the theme
    document.documentElement.classList.add(`${theme}-theme`);
  }

</script>

<DropdownMenu>
  <svg slot="icon" width="37" height="37" viewBox="0 0 37 37" fill="none" xmlns="http://www.w3.org/2000/svg">
    <path d="M29 18.5C29 24.299 24.299 29 18.5 29C12.701 29 8 24.299 8 18.5C8 12.701 12.701 8 18.5 8C24.299 8 29 12.701 29 18.5Z" stroke="currentColor" stroke-width="2"/>
    <path d="M9 28L6.17157 30.8284" stroke="currentColor" stroke-width="2" stroke-linecap="round"/>
    <path d="M1 18.5H5" stroke="currentColor" stroke-width="2" stroke-linecap="round"/>
    <path d="M9 9L6.17157 6.17157" stroke="currentColor" stroke-width="2" stroke-linecap="round"/>
    <path d="M18.5 5L18.5 1" stroke="currentColor" stroke-width="2" stroke-linecap="round"/>
    <path d="M28 9L30.8284 6.17157" stroke="currentColor" stroke-width="2" stroke-linecap="round"/>
    <path d="M32 18.5H36" stroke="currentColor" stroke-width="2" stroke-linecap="round"/>
    <path d="M28 28L30.8284 30.8284" stroke="currentColor" stroke-width="2" stroke-linecap="round"/>
    <path d="M18.5 36L18.5 32" stroke="currentColor" stroke-width="2" stroke-linecap="round"/>
    <circle cx="14.5" cy="18.5" r="6.5" stroke="currentColor" stroke-width="2"/>
    <line x1="8.03459" y1="22.0512" x2="17.227" y2="12.8588" stroke="currentColor" stroke-width="2"/>
    <line x1="11.0346" y1="25.0512" x2="20.227" y2="15.8588" stroke="currentColor" stroke-width="2"/>
  </svg>  

  <div slot="dropdown">
    {#each THEMES as themeOption}
      <button 
        class="theme-button" 
        class:selected={$theme === themeOption.value}
        on:click={() => $theme = themeOption.value}
      >
        <svg width="24px" height="24px" viewBox="0 0 24 24" fill="none" xmlns="http://www.w3.org/2000/svg">
          <circle cx="12" cy="12" r="7" fill="{$theme === themeOption.value ? 'var(--primary-60)' : 'transparent'}"/>
        </svg>
        <span>{themeOption.label}</span>
      </button>
    {/each}
  </div>
</DropdownMenu>

<style>
  :global(svg) {
    color: var(--neutral-70);
  }

  .theme-button.selected {
    font-weight: bold;
  }

  button {
    margin: 0;
    padding: 0;
    border: none;
    border-radius: 4px;
    background-color: transparent;
    color: inherit;
    cursor: pointer;
  }
</style>

When the component is loaded in the browser, we check to see if the user has previously selected a theme. If not it defaults to the automatic theme. The automatic theme sets the theme to either light or dark using the media query prefers-color-scheme to detect if the user has requested light or dark color themes in their OS.

When setting a theme, for instance warm, the class warm-theme is added to the root element of the page, which will override the CSS color variables to the ones we previously defined in the .warm-theme selector in our global.css file.

Adding the theme selector:

Now we can add the ThemeSelector component to our layout file. Change the content of the src/routes/ layout.svelte file to this:

<script>
  import '../styles/global.css';
  import ThemeSelector from '$lib/components/ThemeSelector.svelte';
</script>

<div>
  <header>
    <ThemeSelector />
  </header>
  <main>
    <slot />
  </main>
</div>

<style>
  header {
    display: flex;
    justify-content: flex-end;
  }

  main {
    margin: 50px 0;
  }
</style>

This will add the theme selector dropdown to the top right of all pages.

Add some example content to view the themes:

In the src/routes/ page.svelte file we can add some boxes to view the color themes:

<h1>Theme selector</h1>

<div class="box-container">
  <div class="box neutral-0">--neutral-0</div>
  <div class="box neutral-10">--neutral-10</div>
  <div class="box neutral-30">--neutral-30</div>
  <div class="box neutral-40">--neutral-40</div>
  <div class="box neutral-60">--neutral-60</div>
  <div class="box neutral-70">--neutral-70</div>
  <div class="box neutral-100">--neutral-100</div>
  <div class="box primary-60">--primary-60</div>
  <div class="box primary-70">--primary-70</div>
  <div class="box primary-80">--primary-80</div>
  <div class="box secondary-70">--secondary-70</div>
  <div class="box secondary-80">--secondary-80</div>
</div>

<style>
  .box-container {
    display: flex;
    gap: 30px;
    flex-wrap: wrap;
  }

  .box {
    width: 150px;
    height: 150px;
    display: flex;
    align-items: center;
    justify-content: center;
  }

  .neutral-0 {
    background-color: var(--neutral-0);
    color: var(--neutral-100);
  }

  .neutral-10 {
    background-color: var(--neutral-10);
    color: var(--neutral-100);
    border: 5px solid var(--neutral-30);
    box-sizing: border-box;
  }

  .neutral-30 {
    background-color: var(--neutral-30);
    color: var(--neutral-100);
  }

  .neutral-40 {
    background-color: var(--neutral-40);
    color: var(--neutral-100);
  }

  .neutral-60 {
    background-color: var(--neutral-60);
    color: var(--neutral-0);
  }

  .neutral-70 {
    background-color: var(--neutral-70);
    color: var(--neutral-0);
  }

  .neutral-100 {
    background-color: var(--neutral-100);
    color: var(--neutral-0);
  }

  .primary-60 {
    background-color: var(--primary-60);
    color: var(--neutral-0);
  }

  .primary-70 {
    background-color: var(--primary-70);
    color: var(--neutral-0);
  }

  .primary-80 {
    background-color: var(--primary-80);
    color: var(--neutral-0);
  }

  .secondary-70 {
    background-color: var(--secondary-70);
    color: var(--neutral-0);
  }

  .secondary-80 {
    background-color: var(--secondary-80);
    color: var(--neutral-0);
  }
</style>

And here is the final result

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