Should The CSS light-dark() Function Support More Than Light and Dark Values?

The Evolution of CSS Theming

The light-dark() CSS function represents a significant advancement in native theming capabilities. Introduced as part of CSS Color Module Level 5 and achieving Baseline status in 2024, this function enables developers to specify two color values--one for light mode and one for dark mode--within a single CSS declaration. MDN Web Docs

As web applications become more sophisticated and user expectations evolve, the question arises: should light-dark() support more than just two color scheme values? This exploration examines the current state, proposed solutions, and best practices for modern CSS theming.

The Current State of CSS light-dark()

How light-dark() Works

The light-dark() function operates by detecting the current color scheme preference, which users can set through their operating system's settings or browser preferences. When a user prefers light mode, the first color value is applied; when dark mode is preferred, the second color value takes effect.

Basic syntax:

:root {
 color-scheme: light dark;
}

.element {
 background-color: light-dark(#ffffff, #1a1a2e);
 color: light-dark(#1a1a1a, #f0f0f0);
}

Browser Support and Implementation

As of 2024, light-dark() enjoys broad browser support across Chrome, Firefox, Safari, and Edge. MDN Web Docs This widespread availability has led to rapid adoption in modern web projects. For teams requiring support for older browsers, build tools like LightningCSS and PostCSS plugins provide transpilation capabilities.

The function accepts any valid CSS color value for both arguments, including named colors, hex values, RGB and HSL notations, and CSS custom properties. This flexibility allows developers to reference predefined theme variables or use inline color values depending on their architectural preferences. Understanding how CSS selectors work is complementary knowledge that helps when implementing sophisticated theme-aware styling patterns.

The Case for Extending Beyond Two Values

Accessibility and High Contrast Modes

Many users with visual impairments rely on high contrast modes to make content more readable. Current CSS provides the prefers-contrast media query, but integrating this with light-dark() requires additional complexity that the function's current two-value design doesn't accommodate. 12 Days of Web

For applications serving diverse user populations, supporting a high contrast variant alongside traditional light and dark modes represents a meaningful accessibility improvement.

Custom Brand Themes

Modern web applications often offer users the ability to customize their interface beyond system preferences. Gaming platforms, content management systems, and creative tools provide multiple theme options based on personal preference. CSS-Tricks The current two-value limitation of light-dark() forces developers to layer additional logic on top of the function.

Regional and Contextual Themes

Some applications benefit from themes beyond light and dark--sepia tones for reading, high-visibility themes for accessibility, or brand-specific color schemes. A reading application might offer sepia tones optimized for extended reading sessions. The binary model doesn't represent the full spectrum of user needs.

The W3C Proposal: schemed-value()

The CSS Working Group has been actively discussing the limitations of the two-value approach and the need for a more flexible solution. CSS-Tricks The proposed schemed-value() function would extend the capabilities of light-dark() to support an arbitrary number of color scheme values.

The @color-scheme At-Rule

The proposed @color-scheme at-rule would allow developers to define custom color schemes with specific semantic mappings:

@color-scheme high-contrast {
 --accent: #000000;
 --background: #ffffff;
 --text: #000000;
}

@color-scheme sepia {
 --accent: #704214;
 --background: #f4ecd8;
 --text: #5b4636;
}

.element {
 color: schemed-value(high-contrast black, sepia #5b4636, light #333, dark #f0f0f0);
}

This approach maintains light-dark() simplicity while providing flexibility for advanced theming needs. By registering custom schemes with the browser, developers can reference them in schemed-value() alongside or instead of the built-in light and dark schemes.

Alternative Approaches: CSS Custom Functions

While awaiting broader support for multi-scheme solutions, developers can use CSS custom functions with the if() conditional function for immediate multi-scheme capabilities. CSS-Tricks

@function --theme-color(--scheme, --light-value, --dark-value, --contrast-value) {
 result: if(style(--scheme: dark): var(--dark-value) ; else if(style(--scheme: high-contrast): var(--contrast-value) ; else: var(--light-value));
}

.element {
 color: --theme-color(--scheme, #333, #f0f0f0, #000);
}

This approach requires more setup but provides immediate access to multi-scheme logic without waiting for browser implementations of schemed-value(). For teams using custom Tailwind CSS configurations, these techniques can be integrated into existing workflows. Similarly, extending Sass with PostCSS can help manage complex theme architectures across large codebases.

Performance Considerations

Native vs JavaScript Theming

Native CSS theming through light-dark() applies correct colors before the first paint, eliminating FOIT scenarios. This performance advantage is particularly significant for users on lower-powered devices or in regions with unreliable network connectivity.

Multi-Scheme Performance

Extended theming functions would maintain this performance advantage, as the browser would continue handling theme selection at the CSS parsing level. The proposed schemed-value() function is designed with performance requirements in mind, ensuring custom schemes receive the same efficient treatment as built-in light and dark modes.

Build Tool Optimization

For projects requiring older browser support, build tools like LightningCSS can transpile advanced CSS theming features into compatible output. Extending Sass with PostCSS can further enhance compatibility across different browser versions. Understanding valid CSS content patterns ensures your theme declarations adhere to standards that build tools can reliably process.

Best Practices for Current Implementation

Centralized Theme Definitions

Group all theme-related custom properties in :root for a single source of truth:

:root {
 color-scheme: light dark;
 
 --theme-accent: light-dark(#0066cc, #4da6ff);
 --theme-background: light-dark(#ffffff, #1a1a2e);
 --theme-text: light-dark(#1a1a1a, #f0f0f0);
 --theme-border: light-dark(#e0e0e0, #404060);
}

Semantic Property Naming

Use descriptive names that indicate color roles rather than specific values, enabling easier theme evolution and supporting future extensions. Properties that change based on color scheme receive theme-aware values through light-dark(), while truly universal properties maintain consistent values regardless of theme. This approach aligns with best practices for how CSS selectors work to create maintainable stylesheets that scale with project complexity.

Preparing for Future Extensions

Structure CSS with semantic custom properties that will facilitate migrations to multi-scheme architectures as browser support evolves. Consider how element sizing and spacing interact with theme colors--techniques like CSS height by percentage work alongside theming to create cohesive responsive designs.

Accessibility Implications

Contrast Requirements

When creating themes, ensure sufficient color contrast between foreground and background elements. WCAG guidelines specify minimum contrast ratios that apply regardless of the user's chosen theme.

Combining with prefers-contrast

The prefers-contrast media query detects users who have requested increased contrast through system settings. Combining this with light-dark() requires careful consideration of how contrast preferences interact with color scheme preferences.

Explicit User Controls

For comprehensive accessibility, consider offering dedicated high contrast themes that users can explicitly select, regardless of their system color scheme preference. This approach acknowledges that system-level preferences don't always capture the full range of user needs and provides explicit controls for users who require specific configurations.