Fix small typo in CSS variable name

[4levels]

As explained in issue #367 this is a tiny commit that just adds the missing -- prefix for a CSS variable.

Type

What types of changes does your code introduce?
Fix a tiny typo in a CSS variable name, doesn't need a custom test imho.

Put an x in the boxes that apply_

• Fix
• Feature

Checklist

Put an x in the boxes that apply._

• Title as described
• Add unit test by vitest if necessary

[FahriDevZ]

Thank you for this fix 🙏

Just to add some context:
this typo causes a hard build failure when using lightningcss, since it
strictly validates CSS syntax and does not allow invalid var() declarations.

In my case, this completely blocks the build, so having this fix merged is
really important for projects that rely on lightningcss.

Appreciate the work on this PR — it would be great to see it merged soon.

[agattaollamast]

PLEASE merge this ASAP, same issue about "hard build failure when using lightningcss", this is used by default in vite v8, this issue is blocking build!

[YIllmann]

@HC200ok