CSS3 variables has easily become one of my new favorite features added to CSS. Combining this with HTML5's custom data attributes, we can define entire themes with ease with just a few CSS variables.

What once would have taken an incredible amount of CSS and and JS, we can now do with a CSS and a simple attribute on the body tag.

In addition to themeing, this is useful for responsiveness, and makes organizing our CSS a breeze.

Below we can see two variables defined for an example theme of light/dark, and then setting that variables for the body class.

body[data-theme="light"] {
    --bg-color: white;
}
body[data-theme="dark"] {
    --bg-color: #292828;
}
body {
    background-color: --bg-color;
}

This is an example of how CSS variables are also extremely useful for responsiveness, allowing us to adjust font sizes, spacing on elements, adjusting our header height - just to name a few options.

html {
    --base-font-size: 18px;
    --logo-font-size: 70px;
}

// Make  things smaller for mobile
@media screen and (max-width: 650px) {
    html {
        --base-font-size: 17px;
        --logo-font-size: 60px;
    }
    body {
        --space: 1.5rem;
        --header-height: 60px;
    }
}