defineTheme color config: accent required for neutral-only, lacks light/dark support

[cixzhang]

Problem

The color field in defineTheme has a few ergonomic and type issues:

1. accent is required even when you only want neutralStyle

XDSColorScaleConfig defines accent: string as required. If you just want to set neutralStyle: "warm" without changing the accent color, you still have to pass an accent value:

// This should work but doesnt — accent is required
defineTheme({
  name: "my-theme",
  color: { neutralStyle: "warm" },
});

2. accent only accepts a single string — no light/dark tuple support

The rest of the token system supports [light, dark] tuples via XDSTokenValue, but accent in the color config is typed as just string. This prevents setting a different accent per color scheme:

// Not possible today:
color: { accent: ["#0064E0", "#48CAE4"] }

// Have to use this workaround in tokens instead:
tokens: { "--color-accent": ["#0064E0", "#48CAE4"] }

But that workaround skips the full color scale generation that expandColorScale provides (muted variants, on-accent, text-accent, icon-accent, etc).

3. Unclear light/dark behavior in the color extractor

expandColorScale already generates light-dark() values from a single accent (picking different tonal palette stops for light vs dark). But if a consumer provides both an accent via color.accent AND token overrides via tokens["--color-accent"] as a tuple, the interaction is unclear:

• Does the color extractor generate its full derived palette from the single accent, then get partially overridden by the tuple token?
• Will derived tokens (e.g. --color-accent-muted, --color-on-accent) still be based on the original single accent while --color-accent itself uses the tuple?

This creates potential inconsistency where the accent and its derived variants diverge.

Suggested fixes

1. Make accent optional in XDSColorScaleConfig — allow neutral-only configuration
2. Accept string | [light: string, dark: string] for accent (matching XDSTokenValue)
3. When given a tuple, derive light palette from the light accent and dark palette from the dark accent
4. Document the precedence between color config and tokens overrides for accent-derived values

References

• packages/core/src/theme/expandColorScale.ts — XDSColorScaleConfig type
• packages/core/src/theme/defineTheme.ts — color config integration

[ernestt]

I think @rubyycheung will be expanding on XDSColorScaleConfig with her explorations in Color Studio. I think the idea was to add more controls like grey tone controls and saturation controls. All of which needs to be incorporated with the auto accent color picking.