You can use global custom properties in Houdini paint workers by mapping them to initial values.
Here, I'm mapping the global --border-thin (declared on :root) to the registered --cross-width. The thickness of the cross changes with --border-thin, keeping them in sync.
There's no need to use `--cross-width: var(--border-thin)` on the element. It passes the initial value silently. #CSS
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