[v4] override theme vars in Vue SFC style section

[davidmatter]

Would be nice if we could override theme values in Vue SFC style sections so we can import external components and override their resolved theme var values.

<script setup lang="ts">
import {Card} from "@some-external-package"
</script>

<template>
  <Card></Card>
</template>

<style>
@theme {
    --color-card: #000;
}
</style>