DEV Community

Cover image for A practical CSS variable setup for light/dark mode without theme flash
Hasan Sarwer
Hasan Sarwer

Posted on

A practical CSS variable setup for light/dark mode without theme flash

#css #webdev #darkmode #frontend

Dark mode is easy to start.

It is harder to ship cleanly.

Most implementations need to handle:

  • light mode
  • dark mode
  • system preference
  • stored user preference
  • a toggle button
  • no flash of the wrong theme on page load

The flash is the annoying part.

You save the user's choice in localStorage, but your app JavaScript usually runs after the browser has already started parsing HTML and CSS.

So the page may briefly render in the wrong theme before your JavaScript applies the saved preference.

This article shows a practical setup using:

  • CSS variables
  • data-theme on <html>
  • prefers-color-scheme
  • localStorage
  • a tiny inline script in <head>

No framework required.

The goal

I want components to use semantic variables like this:

body {
  background: var(--color-background);
  color: var(--color-text);
}

.card {
  background: var(--color-surface);
  border: 1px solid var(--color-border);
}

.button {
  background: var(--color-primary);
  color: var(--color-on-primary);
}
Enter fullscreen mode Exit fullscreen mode

The component should not care whether the app is currently light or dark.

Only the variable values should change.

Define light mode variables

Start with light mode as the default.

:root {
  color-scheme: light;

  --color-background: #ffffff;
  --color-surface: #f8fafc;
  --color-text: #0f172a;
  --color-muted: #64748b;
  --color-border: #e2e8f0;

  --color-primary: #2563eb;
  --color-on-primary: #ffffff;
}
Enter fullscreen mode Exit fullscreen mode

This means that if nothing else happens, the site renders in light mode.

That is a safe default.

Add explicit dark mode

Now add dark mode using data-theme="dark" on the root element.

:root[data-theme="dark"] {
  color-scheme: dark;

  --color-background: #020617;
  --color-surface: #0f172a;
  --color-text: #f8fafc;
  --color-muted: #94a3b8;
  --color-border: #1e293b;

  --color-primary: #60a5fa;
  --color-on-primary: #020617;
}
Enter fullscreen mode Exit fullscreen mode

When this attribute exists:

<html data-theme="dark">
Enter fullscreen mode Exit fullscreen mode

the dark variables override the light variables.

Add system preference support

Users may have an OS-level preference for dark mode.

You can respect that with prefers-color-scheme.

@media (prefers-color-scheme: dark) {
  :root:not([data-theme="light"]) {
    color-scheme: dark;

    --color-background: #020617;
    --color-surface: #0f172a;
    --color-text: #f8fafc;
    --color-muted: #94a3b8;
    --color-border: #1e293b;

    --color-primary: #60a5fa;
    --color-on-primary: #020617;
  }
}
Enter fullscreen mode Exit fullscreen mode

The important part is this selector:

:root:not([data-theme="light"])
Enter fullscreen mode Exit fullscreen mode

It means:

Use system dark mode unless the user explicitly selected light mode.

So the behavior becomes:

User choice Result
No saved choice + OS light light
No saved choice + OS dark dark
Saved light light
Saved dark dark

Prevent the wrong theme from flashing

This is the critical piece.

Add a small inline script in <head> before your theme CSS.

<script>
  (function () {
    try {
      var storedTheme = localStorage.getItem('theme');

      if (storedTheme === 'light' || storedTheme === 'dark') {
        document.documentElement.setAttribute('data-theme', storedTheme);
      }
    } catch (_) {}
  })();
</script>
Enter fullscreen mode Exit fullscreen mode

Do not run this script after the page loads.

Avoid:

<script defer src="/theme.js"></script>
Enter fullscreen mode Exit fullscreen mode

Avoid:

document.addEventListener('DOMContentLoaded', ...)
Enter fullscreen mode Exit fullscreen mode

The script should run immediately while the browser is still parsing the <head>.

Why?

Because the browser reads the document top to bottom.

If this script sets data-theme="dark" before the CSS is applied, the correct variables are active before the first paint.

That prevents the wrong theme from flashing.

Recommended HTML structure

Put the inline script before the CSS.

<!doctype html>
<html lang="en">
  <head>
    <script>
      (function () {
        try {
          var storedTheme = localStorage.getItem('theme');

          if (storedTheme === 'light' || storedTheme === 'dark') {
            document.documentElement.setAttribute('data-theme', storedTheme);
          }
        } catch (_) {}
      })();
    </script>

    <style>
      :root {
        color-scheme: light;

        --color-background: #ffffff;
        --color-surface: #f8fafc;
        --color-text: #0f172a;
        --color-muted: #64748b;
        --color-border: #e2e8f0;

        --color-primary: #2563eb;
        --color-on-primary: #ffffff;
      }

      :root[data-theme="dark"] {
        color-scheme: dark;

        --color-background: #020617;
        --color-surface: #0f172a;
        --color-text: #f8fafc;
        --color-muted: #94a3b8;
        --color-border: #1e293b;

        --color-primary: #60a5fa;
        --color-on-primary: #020617;
      }

      @media (prefers-color-scheme: dark) {
        :root:not([data-theme="light"]) {
          color-scheme: dark;

          --color-background: #020617;
          --color-surface: #0f172a;
          --color-text: #f8fafc;
          --color-muted: #94a3b8;
          --color-border: #1e293b;

          --color-primary: #60a5fa;
          --color-on-primary: #020617;
        }
      }
    </style>
  </head>

  <body>
    <button id="theme-toggle" type="button">
      Toggle theme
    </button>
  </body>
</html>
Enter fullscreen mode Exit fullscreen mode

Add the toggle function

Now add a simple toggle.

function toggleTheme() {
  var html = document.documentElement;
  var currentTheme = html.getAttribute('data-theme');

  var nextTheme = currentTheme === 'dark' ? 'light' : 'dark';

  html.setAttribute('data-theme', nextTheme);
  localStorage.setItem('theme', nextTheme);
}

document
  .getElementById('theme-toggle')
  ?.addEventListener('click', toggleTheme);
Enter fullscreen mode Exit fullscreen mode

Now clicking the button switches between:

<html data-theme="light">
Enter fullscreen mode Exit fullscreen mode

and:

<html data-theme="dark">
Enter fullscreen mode Exit fullscreen mode

Add a system mode option

A good theme switcher often has three options:

  • light
  • dark
  • system

For system mode, remove the attribute and clear localStorage.

function resetToSystemTheme() {
  document.documentElement.removeAttribute('data-theme');
  localStorage.removeItem('theme');
}
Enter fullscreen mode Exit fullscreen mode

Now the browser falls back to:

@media (prefers-color-scheme: dark) {
  :root:not([data-theme="light"]) {
    /* dark variables */
  }
}
Enter fullscreen mode Exit fullscreen mode

Why color-scheme matters

This line is easy to forget:

color-scheme: light;
Enter fullscreen mode Exit fullscreen mode

and:

color-scheme: dark;
Enter fullscreen mode Exit fullscreen mode

It tells the browser which color scheme your page supports.

That affects built-in UI such as:

  • form controls
  • scrollbars
  • default input styling
  • browser-rendered UI pieces

So your page does not only change your custom CSS variables. Native browser UI also gets a better matching appearance.

Component CSS stays simple

Once the variables are defined, component CSS becomes boring.

That is good.

body {
  margin: 0;
  background: var(--color-background);
  color: var(--color-text);
  font-family: system-ui, sans-serif;
}

.card {
  background: var(--color-surface);
  border: 1px solid var(--color-border);
  border-radius: 12px;
  padding: 1rem;
}

.card-muted {
  color: var(--color-muted);
}

.button {
  background: var(--color-primary);
  color: var(--color-on-primary);
  border: 0;
  border-radius: 8px;
  padding: 0.625rem 1rem;
  cursor: pointer;
}
Enter fullscreen mode Exit fullscreen mode

There is no separate .dark .button rule.

The variables handle the mode.

The limitation

This setup solves the theme switching structure.

It does not solve color design for you.

You still need to decide:

  • which light colors to use
  • which dark colors to use
  • whether text has enough contrast
  • what hover/pressed/disabled states should be
  • what color should go on top of primary buttons
  • how semantic colors like success/warning/danger behave in both modes

For a small project, manually writing variables may be enough.

For a larger system, it becomes repetitive.

Generating the variables instead

This is why I started working on salt-theme-gen.

Instead of manually maintaining separate light and dark token files, you can generate both modes from one call:

import { generateTheme } from 'salt-theme-gen';

const theme = generateTheme({ preset: 'ocean' });
Enter fullscreen mode Exit fullscreen mode

That returns:

theme.light;
theme.dark;
Enter fullscreen mode Exit fullscreen mode

Each mode has the same structure:

theme.light.colors;
theme.light.states;
theme.light.spacing;
theme.light.radius;
theme.light.fontSizes;
theme.light.accessibility;
Enter fullscreen mode Exit fullscreen mode

So the CSS variable setup can be generated instead of handwritten.

For example:

function kebab(str: string): string {
  return str.replace(/([A-Z])/g, '-$1').toLowerCase();
}

function modeToVars(mode: typeof theme.light): string {
  const lines: string[] = [];

  for (const [key, value] of Object.entries(mode.colors)) {
    lines.push(`  --color-${kebab(key)}: ${value};`);
  }

  for (const [key, value] of Object.entries(mode.spacing)) {
    lines.push(`  --space-${key}: ${value}px;`);
  }

  for (const [key, value] of Object.entries(mode.radius)) {
    lines.push(`  --radius-${key}: ${value}px;`);
  }

  for (const [key, value] of Object.entries(mode.fontSizes)) {
    lines.push(`  --text-${key}: ${value}px;`);
  }

  return lines.join('\n');
}
Enter fullscreen mode Exit fullscreen mode

Then:

const themeCSS = `
:root {
  color-scheme: light;
${modeToVars(theme.light)}
}

:root[data-theme="dark"] {
  color-scheme: dark;
${modeToVars(theme.dark)}
}
`;
Enter fullscreen mode Exit fullscreen mode

The browser still uses the same CSS variable strategy.

The difference is that the design tokens are generated from a system instead of being maintained by hand.

The bottom line

A reliable light/dark setup does not need to be complicated.

The key pieces are:

CSS variables for tokens
data-theme on <html>
prefers-color-scheme for system fallback
localStorage for user preferences
inline script in <head> to avoid theme flash
Enter fullscreen mode Exit fullscreen mode

The most important rule:

Set the saved theme before the CSS is applied.

After that, your components can stay clean:

background: var(--color-background);
color: var(--color-text);
Enter fullscreen mode Exit fullscreen mode

The values change between light and dark mode.

The component contract stays the same.

Top comments (0)