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name: color-mode-and-theme description: Light, dark or both, and when a theme selector is worth it. Use when defining a base palette or deciding on dark mode support. metadata: priority: 7 pathPatterns: - "/*.css" - "/.scss" - "/tokens/" - "/theme/" - "tailwind.config." - "design-system/**" promptSignals: phrases: - "dark mode" - "light mode" - "theme" - "color mode" - "dark design" - "white design" - "theme selector" - "brand tone" retrieval: aliases: - dark mode - light mode - theme selector - color scheme - brand tone - white design - dark design intents: - choose between dark and light mode - decide if dark mode is needed - build a theme selector - match brand tone in color palette - design for enterprise tools examples: - should this app be dark or light - when should I add a theme toggle - design a dark mode for this dashboard - this is a trading platform, what color mode fits

Color Mode and Theme

The Decision: Light, Dark, or Both

Color mode is a brand and context decision, not a personal preference. Make it deliberately.

Light (white design)

Tone: Open, trustworthy, content-forward, accessible, professional
Fits: Marketing sites, e-commerce, editorial, SaaS with mixed audiences, consumer products, B2B tools where the content is the focus

Light mode is the safer default for most products. It performs better in bright environments and has broader accessibility coverage out of the box.

Dark (dark design)

Tone: Premium, focused, immersive, technical, high-contrast data
Fits: Trading platforms, developer tools, creative tools (video/audio editors), data dashboards with dense visualisations, entertainment, gaming

Dark mode reduces eye strain during extended use in low-light environments. It also makes colourful data visualisations (charts, heatmaps) pop more clearly against a dark surface.

Caution: Dark mode is harder to get right. Low-contrast text, over-saturated brand colours, and insufficient surface differentiation are common failures. If the team cannot maintain it properly, light mode is better than a broken dark mode.

Combined (system-default + manual override)

Respect prefers-color-scheme and let the OS set the default. Offer a toggle for users who want to override. This is the modern standard for most products with a returning user base.

@media (prefers-color-scheme: dark) {
  :root { /* dark tokens */ }
}
@media (prefers-color-scheme: light) {
  :root { /* light tokens */ }
}
[data-theme="dark"] { /* manual override */ }
[data-theme="light"] { /* manual override */ }

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When to Build a Theme Selector in the UI

A theme selector is a UI control — it takes space and adds complexity. Only build it when user control genuinely matters.

Situation	Theme selector?
Marketing site, landing page	No — pick one mode, commit to it
Consumer SaaS, general audience	Combined (system-default + toggle)
Developer tool, technical product	Yes — developers expect it
Trading platform, financial dashboard	Yes — extended use, low-light sessions common
B2B enterprise tool (ERP, analytics)	Yes — power users, long sessions, personal preference varies
E-commerce storefront	Usually no — light default, possibly system-default
Creative tool (design, video, audio)	Yes — dark is often preferred, toggle still expected

Rule: if the user will spend hours per day in the tool, give them control. If it's a transactional or occasional-use product, pick the mode that fits the brand and move on.

Place the theme toggle in the header (top-right, near account) or in user settings — not in primary navigation.

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Brand Tone and Color Mode

The brand's existing visual identity should inform the default mode.

Brand tone	Default mode
Clean, minimal, trustworthy, open	Light
Premium, exclusive, bold, immersive	Dark
Technical, data-heavy, precise	Dark
Playful, colourful, energetic	Light (dark mode harder to maintain with vivid brand colours)
Neutral, enterprise, functional	Light, with system-default toggle

If the brand uses a very dark primary colour (navy, deep green, near-black), a dark mode surfaces it naturally. If the brand is built around a bright, vivid primary, light mode lets it breathe.

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Dark Mode Token Principles

A theme switcher is a second palette, not a toggle. Every semantic colour needs a verified value in each mode — not one value flipped by an algorithm. Contrast that passes on light routinely fails on dark; saturation and elevation read differently. Real cost, and a reason not to build one unless it earns its place (see above).

Dark mode is not just inverting colours. Common mistakes:

• Do not use pure black (#000000) as the base surface — use a very dark neutral (#0A0A0F, #111827) for depth
• Surface hierarchy in dark mode uses lightness, not shadows — base, +1, +2 surfaces get progressively lighter, not more shadowed
• Reduce brand colour saturation slightly — vivid colours on dark backgrounds can be visually aggressive; a 10–15% desaturation keeps them readable
• Increase font weight for reversed text — light text on a dark background often appears "thinner" than the same weight in light mode. Increase the weight by one step (e.g., from Regular to Medium, or Medium to Semibold) to maintain legibility.
• Text contrast needs active verification — light text on dark surfaces is not automatically WCAG-compliant; check all combinations

A literal equal to a token's value is that token. When a hardcoded colour happens to match what a token resolves to in the current theme, it is not a coincidence and not a local choice: it is that token, written in the one form that cannot follow the theme. The damage then shows up inverted rather than missing. A surface frozen at its dark-mode value does not merely look wrong in light mode, it reverses its relationship to its neighbours, so what was sunken becomes raised and what receded now advances. The layout still renders, nothing throws, and the only report is somebody saying a screen feels off. Do not leave this to discipline, because a literal is invisible to every token audit by construction. Make it mechanical: grep the markup for literal colour values, treat the count as a debt with a number, and fail the build on new ones once it reaches zero. Any literal that survives the grep needs a reason in writing, and "it is the same as the token anyway" is the one reason that is never valid.

/* Dark mode surface scale */
--color-surface:         #0A0A0F;  /* base */
--color-surface-raised:  #141419;  /* cards */
--color-surface-overlay: #1E1E26;  /* modals, popovers */
--color-border:          rgba(255,255,255,0.08);
--color-text:            #F0F0F5;
--color-text-secondary:  #8A8F98;

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Review Checklist

• Is the color mode choice deliberate and based on brand tone and use context?
• If combined mode: does prefers-color-scheme set the default?
• If a theme selector is built: is it placed in a low-profile but accessible location?
• In dark mode: are surfaces differentiated by lightness steps, not just shadows?
• In dark mode: is brand colour slightly desaturated to avoid visual aggression?
• Are all text/background contrast ratios verified in both modes (WCAG 2.2 AA)?
• Is font weight increased by one step for reversed text (light-on-dark) to maintain optical legibility?
• Is pure black (#000000) avoided as a dark mode base surface?