perf(preset-mini): apply preflight vars to :root

[antfu]

I don't know why we aren't doing it from the beginning 🤣

/cc @chu121su12 in case you are aware of any caveat of this approach.

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[sibbng]

This change turns per element variables into some kind of global variables. As a result, nested elements will have unwanted styles. For example:

<div class="p-10 bg-black text-white">
  <div class="ring-8 ring-offset-8 p-4">
    should have offset
    <div class="ring transform">shouldnt have offset</div>
  </div>
</div>

UnoCSS Playground - Tailwind Playground

Expected (Tailwind Playground): Nested div shouldn't have ring offset

Actual (UnoCSS): Nested div have ring offset

This issue will appear for all CSS variables that are used in the output but do not have a redefinition, such as ring-color, ring offset color, shadow utilities etc.

[antfu]

Oh I see. Thanks for the explanation and I will revert it