Opacity modifiers on theme colours compile to color-mix(unset) on web and native #705

Opacity modifiers on theme colours compile to color-mix(… unset …) on web and native

Versions: uniwind 1.12.0 and 1.12.1, tailwindcss 4.3.3, Expo SDK web and Android export.

Reproduction

global.css:

@import "tailwindcss";
@import "uniwind";

@layer theme {
  :root {
    @variant light {
      --primary: #1f4c96;
    }
    @variant dark {
      --primary: #8fb8ff;
    }
  }
}

@theme inline {
  --color-primary: var(--primary);
}

Any component using className="bg-primary/12".

Expected: the colour mixes the current theme's --primary, as bg-primary does
(background-color: var(--primary)).

Actual:

• Web (expo export -p web): .bg-primary\/12{background-color:color-mix(in srgb, unset 12%, transparent)}.
  Browsers drop the declaration, so the class has no effect.
• Native (expo export -p android --no-bytecode): backgroundColor is
  colorMix("unset", "12%", "#00000000").

The same happens with an arbitrary value, bg-[color-mix(in_srgb,var(--primary)_12%,transparent)].

Likely cause: the themes artifact declares every theme variable as unset in an @theme
block (src/bundler/artifacts/css/themes.ts, `${variable}: unset;`). Tailwind then
resolves var(--primary) against that value when it builds the colour-mix fallback for opacity
modifiers, and Uniwind uses that fallback.

Workaround on web: declaring the mapping with plain @theme instead of @theme inline makes
Tailwind also emit @supports (color: color-mix(in lab, red, red)) { … var(--color-primary) … },
which browsers apply. Native still receives colorMix("unset", …).