Design system

A design system is a short list of tokens in code and one page that says which token to use when. Every screen uses tokens, never raw values. Then a change in one place changes the whole app, and an agent can review a screen against one page.

Start from windows-design. WinUI already ships hundreds of theme resources (TextFillColorPrimaryBrush, ControlCornerRadius). Tokens cover only what WinUI does not: the accent, custom surfaces, spacing, motion, overlays, and custom controls.

The app keeps the Fluent look. Only the accent changes. For parts of your own (a custom navigation rail, buttons, icons), call the Skill tool with design-kit. To copy the look of an app the user names, call it with design-from-app.

The template already has the base in src/<App>/UI/:

• Theme.cs: the accent choices (Theme.All), Theme.ApplyAccent, which writes the seven SystemAccentColor shades WinUI derives every accent brush from, the app theme modes, and Theme.Reapply, which makes a new accent show at once.
• Pages/SettingsPage: Settings > Appearance, where people pick the app theme and the accent. "Use system setting" and "Windows default" are the defaults.
• Space.xaml and Grid.cs: spacing (see the layout-grid skill).

1. Pick the accent

Read ## Look in docs/BRIEF.md. With no color named, keep system: it follows the accent the person set in Windows Settings > Personalization > Colors.

For a brand color, add an AccentChoice to Theme.All with its base color. ApplyAccent derives the light and dark shades: light mode fills with Dark1 and writes accent text with Dark2; dark mode fills with Light2 and writes with Light3. Check that white text on the base reads well; for yellow, mint, or cyan pick a deeper base. Keep Theme.DefaultId = "system" unless the owner asks for the brand by default: Windows people expect their own accent, and the person can still pick the brand in Settings.

Use brand color sparingly: the app icon, an empty state, one hero element in the content. Keep it off most controls.

To compare colors, take snapshots with tools/snap.ps1 -Accent <id> for two or three of them and show the user the PNGs.

Check: Theme.DefaultId is "system" (or the owner chose otherwise in writing), and the snapshots show the accent on the accent button, toggles, and the selection pill in light and dark mode.

2. Collect

New app with no pages yet? Skip this step: write the token files with the defaults below before the first page.

Existing app: read every page. List each raw value you find: hex colors, opacities, corner radii, margins, paddings, widths, animation durations and easing, custom styles. Group them. Most apps have 3 to 5 of each where 1 or 2 are meant.

3. Write the tokens

Accent values live in Theme.cs. The rest get one file per group in src/<App>/UI/, merged in App.xaml. Add each token when the app first needs it:

Brushes adapt to light, dark, and high contrast through ThemeDictionaries. Pages read them with {ThemeResource ...}, never {StaticResource ...}, so a theme change redraws them.

4. Write design/DESIGN-SYSTEM.md

One section per file above. Each section says which token to use when, and the rules that go with it. Keep it under 100 lines. Rules that transfer to most apps:

• Never hard-code a hex accent. Use AccentFillColorDefaultBrush and friends; they follow Theme.ApplyAccent.
• Every custom brush has three theme values: Light, Dark, and HighContrast (which points at a system color like SystemColorWindowTextColor).
• Never reuse a semantic brush for another job (a stroke brush as text, a text brush as a fill). Their values change between releases.
• {ThemeResource} for anything that changes with the theme; {StaticResource} only for numbers, thicknesses, and styles.
• Accent text uses AccentTextFillColorPrimaryBrush. Text on an accent fill uses TextOnAccentFillColorPrimaryBrush, never a fixed white.
• Style controls with Style and, when needed, a ControlTemplate copy with every visual state. Never rebuild a control from shapes: it loses the keyboard, focus, and Narrator.
• Inner corners are concentric: inner radius = outer radius - inset.
• Every animation goes through Motion, so "Animation effects" off and a per-feature "Full, Fast, Off" setting apply everywhere.
• Search results and switchers do not animate. Speed wins.
• Pointer hover never moves a selection. Only a click or a key does.
• Units ("KB/s", "%") never animate. Only the number does.
• Never animate a page's first layout. Overlays wait one frame, then fade and scale in over 167 ms.
• One accent button per page or dialog.

5. Migrate

Replace raw values screen by screen, one commit per screen. After each, build and snapshot (build-and-verify skill). The screen looks the same or better, never worse.

6. Point agents at it

Add one line to the app's CLAUDE.md: "Design rules: design/DESIGN-SYSTEM.md. Use its tokens; never raw colors, radii, or durations."

Done when this finds no hits outside the token files:

Get-ChildItem src -Recurse -Include *.xaml,*.cs |
  Where-Object { $_.Name -notmatch '^(Theme\.cs|Space\.xaml|Grid\.cs|Surfaces\.xaml|Radius\.xaml|Motion\.cs|Tokens\.(xaml|cs))$' } |
  Select-String -Pattern '#[0-9A-Fa-f]{6,8}\b|Color\.FromArgb|Colors\.(White|Black|Gray)|CornerRadius="\d|Duration="0:0:'

(hex strings in data and comments are fine), and DESIGN-SYSTEM.md has a section for every token file.