Theme switcher using CSS custom properties

CSS custom properties have been around for a while now and are currently supported in three browser engines (as of February 2016). When I first heard about CSS variables I was skeptical as I thought they will offer the same functionality as variables in pre-/post-processors. Some month later I stumbled across the spec and was pretty excited to find out that they are not really variables, but rather properties which are dynamic and scoped to the DOM. To show this in action I build a simple theme switcher demo.

Feature detect

As we will use JavaScript to modify custom properties, the first thing we do is adding a detect to decide whether we will show the theme switcher or not.

if (window.CSS && window.CSS.supports && window.CSS.supports('--a', 0)) {
// CSS custom properties supported.
} else {
// CSS custom properties not supported
}

Setting custom properties in CSS

Next we define some custom properties in our CSS. Here we have a –baseColor, –brandColor and –bgColor which we will use for various elements later in the CSS.

:root {
--baseColor: #111;
--brandColor: green;
--bgColor: #eee;
}

h1 {
color: green; /* provide fallback */
color: var(--brandColor);
}

Modifying custom properties with JavaScript

When selecting a theme we will change the properties with JavaScript, as shown here:

var root = document.querySelector(':root');
var htmlStyle = window.getComputedStyle(root);
htmlStyle.getPropertyValue('--baseColor'); // get the custom property --baseColor
root.style.setProperty("--baseColor", "red"); // set --baseColor to red

Full example

Here is the HTML of the theme switcher, which is hidden by default and will be set to visible once the feature detect above returns true.

<label id="theme-changer" hidden>
Choose theme
<select name="theme" id="theme">
<option data-baseColor="#111" data-bgColor="#eee" data-brandColor="green" value="default">default</option>
<option data-baseColor="#444" data-bgColor="white" data-brandColor="#222" value="light">light</option>
<option data-baseColor="white" data-bgColor="black" data-brandColor="#eee" value="dark">dark</option>
</select>
</label>

Here is the full JavaScript for the theme switcher. As you can see I also save the current chosen theme in localStorage so users will get the selected theme on future visits.

// test for localStorage support
var hasLocalStorage = function () {
var mod = 'a';
try {
localStorage.setItem(mod, mod);
localStorage.removeItem(mod);
return true;
} catch (e) {
return false;
}
};

// test for CSS custom properties support
if (hasLocalStorage() && window.CSS && window.CSS.supports && window.CSS.supports('--a', 0)) {

var themeChanger = document.querySelector('#theme-changer'),
root = document.querySelector(':root'),
themeInput = document.querySelector('#theme'),
currentTheme = {
baseColor: localStorage.getItem("baseColor") || '#111',
bgColor: localStorage.getItem("bgColor") || '#eee',
brandColor: localStorage.getItem("brandColor") || 'green'
},
selectOption;


// show the theme changer
themeChanger.classList.remove('hidden');
themeChanger.removeAttribute('hidden');

// change colors and save to localStorage
themeInput.addEventListener('change', function (e) {

// get selected option
selectOption = this.options[this.selectedIndex];

// change values to current theme
currentTheme = {
baseColor: selectOption.getAttribute('data-baseColor'),
bgColor: selectOption.getAttribute('data-bgColor'),
brandColor: selectOption.getAttribute('data-brandColor')
};

// change values of custom properties and save to localStorage
for (var key in currentTheme) {
root.style.setProperty("--" + key, currentTheme[key]);
localStorage.setItem(key, currentTheme[key]);
}
});

// set theme to previous choosen one
window.onload = function(e) {

// set option of current theme to selected
document.querySelector('[data-baseColor="' + currentTheme.baseColor + '"]').selected = true;

// get values of custom properties and apply
for (var key in currentTheme) {
root.style.setProperty("--" + key, currentTheme[key]);
}
}

} else {
// CSS custom properties not supported - don't show the theme switcher
}

You can find the full demo here and the code is also available on Github.

Update 20.02.2016: Change styles based on different className

As Tomáš Kapler points out in the comments you could also change the html class with JavaScript and change the properties based on the className:

:root {
--baseColor: #111;
--brandColor: green;
--bgColor: #eee;
}
:root.light {
--baseColor: #444;
--bgColor: white;
--brandColor: #222;
}
:root.dark {
--baseColor: white;
--brandColor: #eee;
--bgColor: black;
}

Here is a demo using this method.

Conclusion

As you can see, you can already use CSS custom properties as an enhancement today, but you should always provide a fallback and be careful when using them. If you only need static and lexically scoped variables you are probably better of by using a preprocessor until more browsers support custom properties.

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