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