Theming

Light, dark & density

Two orthogonal switches that compose without friction. A dashboard in dark + compact reads as a single intentional design, not two settings stitched together.

Light & dark

Every color token resolves through CSS variables that have a light and a dark value. Switching is a class toggle — .dark on <html> — and next-themesdrops it before paint, so there's no flash.

How tokens are structured

In app/globals.css, light values live under :root and dark values under .dark. Both blocks reference the same token names — don't invent a --color-card-dark; it's the same --card re-bound under .dark.

How a token resolves across modes

:root {
  --background: oklch(0.99 0 78.3);
  --foreground: oklch(0.153 0.006 107.1);
  --card: oklch(0.99 0 78.3);
  --card-foreground: oklch(0.153 0.006 107.1);
  --primary: oklch(0.228 0.013 107.4);
  --primary-foreground: oklch(0.988 0.003 106.5);
}

.dark {
  --background: oklch(0.153 0.006 107.1);
  --foreground: oklch(0.988 0.003 106.5);
  --card: oklch(0.228 0.013 107.4);
  --card-foreground: oklch(0.988 0.003 106.5);
  --primary: oklch(0.93 0.007 106.5);
  --primary-foreground: oklch(0.228 0.013 107.4);
}

Theme-agnostic tokens

Some tokens have a single value across both modes — the system shapes them once and relies on the underlying surface to provide the correct contrast. Examples: --ring-width, --radius-*, --shadow-*, --duration-*, --ease-*, --z-index-*, --size-*. Don't put these inside .dark.

Wiring the toggle

import { ThemeProvider } from "next-themes";

export default function RootLayout({ children }: { children: React.ReactNode }) {
  return (
    <html lang="en" suppressHydrationWarning>
      <body>
        <ThemeProvider
          attribute="class"      /* toggles .dark on <html> */
          defaultTheme="system"  /* respect OS */
          enableSystem
          disableTransitionOnChange  /* avoid flicker on switch */
        >
          {children}
        </ThemeProvider>
      </body>
    </html>
  );
}

Testing dark mode

Use resolvedTheme (not theme) when you need to know which mode is actually rendering. theme can be "system".
Status colors (destructive / success / warning / info) keep their hue across themes — only lightness and chroma flip. Don't bind .dark --destructive to a different hue.
Charts: --color-chart-* have dark variants. The brand anchor stays at series 1 in both modes.
color-mix(in oklch, …) in light/dark is fine — the system uses it for status backgrounds and state layers.

Density

Density is the second axis. Toggling data-density="compact" on <html> (or any ancestor) tightens controls and gaps without changing typography or surface roles. It composes with light/dark — a compact dark dashboard reads as one design, not two.

What responds to density

--density-control-h — control heights (buttons, inputs, selects)
--density-card-padding — card padding (compact cards take 75% of the comfortable padding)
--density-field-gap — vertical gap between form fields

Components consuming these tokens (h-(--density-control-h), p-(--density-card-padding), gap-(--density-field-gap)) get density-aware behavior for free. Hardcoded h-10 / p-4 / gap-4 does not.

Scoping

<html lang="en" data-density="compact">
  …everything inside is compact
</html>

When to reach for compact

Data-heavy product UIs (tables, dashboards, admin panels)
Expert tools where information density matters more than reading comfort
Side-by-side comparisons (diff views, schema editors, inspector panels)

When NOT to use compact

Marketing pages, hero sections, top-of-funnel content
Mobile-first surfaces (touch targets stay ≥ --size-touch-target regardless of density)
Onboarding flows or anything where comprehension matters more than scanning

Adding density-awareness to a custom component

If you build a component that takes a height or padding, consume the density token instead of hardcoding. The component then participates in compact mode without any additional work:

function FilterChip({ children, ...props }) {
  return (
    <button
      {...props}
      className="
        h-(--density-control-h) px-3
        rounded-chip border-border
        bg-muted text-foreground-subtle
        hover:bg-state-hover
      "
    >
      {children}
    </button>
  );
}

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