CSS Variables Theme Switcher

Change theme with ease using CSS variables and vanilla JavaScript

by proticm

HTML

<main>
        <aside>
            <h1>Theme Switcher</h1>
            <h3>Defaults</h3>
            <span class="btn vue-green" data-bg-color="#42b883" data-color="#35495e">&nbsp;</span>
            <span class="btn fb-blue" data-bg-color="#3b5998" data-color="#8b9dc3">&nbsp;</span>
            <span class="btn tw-blue" data-bg-color="#0084b4" data-color="#00aced">&nbsp;</span>
            <h3>Font Size</h3>
            <select>
                <option value="12">12px</option>
                <option value="15" selected>15px</option>
                <option value="25">25px</option>
            </select>
            <h3>Dynamic Color</h3>
            <label for="bg-color">Background:</label>
            <input type="color" id="bg-color">
            <label for="color">Color:</label>
            <input type="color" id="color">
        </aside>
        <section>
            <p>
                Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et
                dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip
                ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu
                fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia
                deserunt mollit anim id est laborum.
            </p>
            <p>
                Sed ut perspiciatis unde omnis iste natus error sit voluptatem accusantium doloremque laudantium, totam
                rem aperiam, eaque ipsa quae ab illo inventore veritatis et quasi architecto beatae vitae dicta sunt
                explicabo. Nemo enim ipsam voluptatem quia voluptas sit aspernatur aut odit aut fugit, sed quia
                consequuntur magni dolores eos qui ratione voluptatem sequi nesciunt. Neque porro quisquam est, qui
                dolorem ipsum quia dolor sit amet, consectetur, adipisci velit, sed...

CSS

:root {
    --primary-color: #42b883;
    --primary-bg-color: #35495e;
    --primary-font-size: 15px;
    --color-white: #fff;
}

body {
    transition: background-color 0.5s ease-out;
    background-color: var(--primary-bg-color);
}

main {
    display: flex;
    margin: 20px auto;
    max-width: 1200px;
    transition: color 0.5s ease-out;
    color: var(--primary-color);
}

aside {
    border-right: 1px solid var(--color-white);
    padding-right: 1rem;
    text-align: center;
    flex: 0 0 16.666667%;
    max-width: 16.666667%;
}

section {
    padding: 1rem;
    flex: 0 0 83.333333%;
    max-width: 83.333333%
}

p {
    font-size: var(--primary-font-size);
}

label, input {
    display: block;
    margin: 20px auto
}

.btn {
    border-radius: 50%;
    width: 50px;
    height: 50px;
    margin: 10px auto;
    cursor: pointer;
    border: none;
    display: block;
    border: 1px solid var(--color-white);
}

.btn.vue-green {
    background-color: #35495e;
    color: #42b883
}

.btn.fb-blue {
    background-color: #3b5998;
    color: #8b9dc3
}

.btn.tw-blue {
    background-color: #c0deed;
    color: #0084b4
}

JavaScript

const themeSwitchers = document.querySelectorAll('span');
const fontSwitcher = document.querySelector('select');
const dynamicInputs = document.querySelectorAll('input');

const handleThemeUpdate = (cssVars) => {
    const root = document.querySelector(':root');
    const keys = Object.keys(cssVars);
    keys.forEach(key => {
        root.style.setProperty(key, cssVars[key]);
    });
}

themeSwitchers.forEach((item) => {
    item.addEventListener('click', (e) => {
        handleThemeUpdate({
            '--primary-bg-color': e.target.getAttribute('data-bg-color'),
            '--primary-color': e.target.getAttribute('data-color')
        });
    });
});

fontSwitcher.addEventListener('change', (e) => {
    handleThemeUpdate({
        '--primary-font-size': `${e.target.value}px`
    });
});

dynamicInputs.forEach((item) => {
    item.addEventListener('change', (e) => {
        const cssPropName = `--primary-${e.target.id}`;
        handleThemeUpdate({
            [cssPropName]: e.target.value
        });
    });
});