


Creating Color Themes With Custom Properties, HSL, and a Little calc()
Apr 07, 2025 am 09:37 AMBefore CSS custom properties (often called "variables"), managing multiple color schemes on a single website required separate stylesheets—a cumbersome approach. Now, we can define variables within a single stylesheet and let CSS handle the variations.
Even without user-defined themes, site-wide theming is valuable. For example, different sections might employ distinct color palettes.
Let's build an example: Our example uses a consistent lightness across sections, varying only the hue. A simplified palette for a single hue looks like this: [Example palette omitted for brevity, as it's visually represented in the original]. Multiple hues would expand this, easily managed with HSL where only one value changes.
Leveraging Custom Properties
Widely supported and easily polyfilled for older browsers, custom properties offer a straightforward syntax similar to standard CSS. Basic usage is shown below: [Basic usage example omitted for brevity, as it's conceptually explained in the original]. Variables are commonly defined on the :root
pseudo-element but can be scoped to specific elements using attributes like data attributes.
Integrating calc()
Custom properties aren't limited to fixed values. The calc()
function enables dynamic value calculations within a consistent pattern: [Example using calc() omitted for brevity, as it's conceptually explained in the original]. While CSS lacks loops, preprocessors can assist in generating parts of the code (but remember: CSS variables differ from Sass variables).
Implementing CSS Variables in Practice
Our goal is to change a component's color across different page sections. We'll use three sections with IDs: #food
, #lifestyle
, and #travel
, each associated with a different hue. The data-theme
attribute on a .wrapper
div determines the active hue. When #travel
is active, --first-hue
(e.g., 180° for teal) is assigned to --hue
.
<div data-theme="travel"> <!-- Content --> </div> <style> .wrapper[data-theme="travel"] { --hue: var(--first-hue); /* = 180° = teal */ } </style>
A small JavaScript snippet updates the data-theme
attribute based on tab clicks, removing the hash (#
). This leverages JavaScript's ability to manipulate CSS, unlike preprocessor variables which are static after compilation.
Progressive Enhancement
To ensure accessibility for users with JavaScript disabled, we add progressive enhancement:
// progressive enhancement: // without JavaScript all sections are displayed, the theme is only set when the page loads wrapper.dataset.theme = wrapper.querySelector('section').id;
This ensures basic functionality even without JavaScript.
While a single-page approach is used here, serving sections as separate pages with server-side data-theme
setting is also feasible.
Alternative Approaches
If color values don't change linearly, we can separate stylesheets: [Example omitted for brevity, as it's conceptually explained in the original].
Supporting Web Components
Theming web components requires the :host-context()
pseudo-selector: [Example omitted for brevity, as it's conceptually explained in the original].
Conclusion
CSS custom properties simplify website theming, offering improved maintainability, performance, and integration with JavaScript. Combined with HSL and calc()
, they unlock powerful theming capabilities, extending beyond simple color changes to user-controlled themes.
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