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casooncasoon/ai-agent-config/skills/darkmode/SKILL.md

darkmode

Skill1 starsChanged 46 days agoMITView on GitHub

Dark mode implementation for Astro + Tailwind v4 — class-based toggle, FOUC-free init script, cookie/localStorage persistence, and contrast-safe light/dark token pairing.

The file

We split this skill into the rules it gives an agent

1 rule. About 0 are common, 0 are distinctive, and 1 is very specific to this project.

  • ≈0

    Common · estimate

    Many files like this have a rule of this kind.

  • ≈0

    Distinctive · estimate

    This author's own choices. Worth a look.

  • 1

    Very specific

    Tied to their project's names, files or tools. Probably skip.

  • Common
  • Distinctive
  • Very specific
  1. Very specific:

    Use the Cookie Store API only if your target browsers support it.

    line 83

≈ “Common” is an estimate: at least 1 in 5 of the 350,495 skill files we read have a rule of the same kind, not the same words. “Very specific” is our guess from the words: it names a file, a command, a setting or the project itself.

Keep what fits and drop the rest. Your version, ready for your agents.
Files
SKILL.md

Dark mode

Activation

Class-based (recommended — supports a toggle)

Tailwind v4 custom variant in the CSS entry:

@custom-variant dark (&:where(.dark, .dark *));

FOUC-free init script in <head> — must be is:inline to run before paint:

<script is:inline>
  function getStoredTheme() {
    try {
      return document.cookie.match(/(?:^|;\s*)theme=(dark|light)(?:;|$)/)?.[1]
        || localStorage.getItem('theme');
    } catch (e) {
      return null;
    }
  }

  function applyTheme() {
    var t = getStoredTheme();
    document.documentElement.classList.remove('no-js');
    if (t === 'dark' || (!t && matchMedia('(prefers-color-scheme: dark)').matches)) {
      document.documentElement.classList.add('dark');
    } else {
      document.documentElement.classList.remove('dark');
    }
  }

  applyTheme();
  if (window.__applyThemeHandler) {
    document.removeEventListener('astro:after-swap', window.__applyThemeHandler);
  }
  window.__applyThemeHandler = applyTheme;
  document.addEventListener('astro:after-swap', window.__applyThemeHandler);
</script>

Re-registering on astro:after-swap matters for ClientRouter (SPA transitions) so the theme persists across navigations.

OS-only (no toggle)

Tailwind v4 uses prefers-color-scheme by default — no configuration needed, just use dark: utilities.

Persistence

Cookie

document.cookie = 'theme=' + value + ';path=/;max-age=31536000;SameSite=Lax';

Technical cookie — no consent banner required (UI preference, not tracking). Add domain= only when cross-subdomain persistence is a real requirement.

localStorage fallback

localStorage.setItem('theme', value);

Toggle

btn.addEventListener('click', function () {
  var isDark = document.documentElement.classList.toggle('dark');
  var v = isDark ? 'dark' : 'light';
  document.cookie = 'theme=' + v + ';path=/;max-age=31536000;SameSite=Lax';
  try { localStorage.setItem('theme', v); } catch (e) {}
});

Use the Cookie Store API only if your target browsers support it. Plain document.cookie remains the compatibility path.

CSS patterns

Scoped styles (Astro / Svelte)

<style>
  .card { background: #f8f9fa; color: #1f2937; }
  :global(html.dark) .card { background: #2a3234; color: #d2cdc8; }
</style>

Tailwind utilities

<div class="bg-white dark:bg-slate-800 text-gray-900 dark:text-gray-100">

Color guidelines

PrincipleLightDark
BackgroundWarm off-whiteWarm dark (never pure black)
TextNear-blackOff-white (never pure white)
BordersLight grayMuted dark
AccentsAdjust brightnessLighten or warm up

Contrast minimums (WCAG AA)

  • Body text: 4.5:1.
  • Large text (≥ 18 px bold / 24 px): 3:1.
  • Interactive elements: 3:1.

Both modes must independently meet these — see the accessibility-audit skill for full pairing tables.

Logo handling

<img src="/logo.svg"      class="block dark:hidden" alt="Logo" />
<img src="/logo-dark.svg" class="hidden dark:block" alt="Logo" />

Common mistakes

MistakeFix
#000 backgroundUse warm dark, e.g. #1e2628, #222a2c
#fff text on darkUse off-white, e.g. #e6e1dc, #d2cdc8
Cold grays in dark modeUse warm-tinted grays (teal/sand undertone)
Same accent in both modesLighten or warm the accent on dark surfaces
@media (prefers-color-scheme) inside scoped stylesUse :global(html.dark) for class-based toggle
Forgetting dark:border-*Borders are very visible in dark mode — always pair

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