refactor(core): card setCSS takes CSS builders

[marcalexiei]

• setCSS({ light, dark }) now takes (colors: CardColors) => string builders instead of CSS strings.
  Card holds both palettes, so it calls dark only when it has dark colors.
• Every card drops its dark: darkColors ? … : null ternary and its lightColors destructuring,
  each builder reads the colors of its own scheme from its parameter.
• CardColors moved to common/color.ts and is exported from there; Card.ts had a duplicate 5-field copy.

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