Fix up some copy-pasteoes in the textarea CSS defs.
Review Request #10905 — Created Feb. 17, 2020 and discarded
When the textarea definitions got moved to the new UI namespace, the old
fallback definitions were incorrectly updated and all pointed to the
same variable. This change fixes it up.
Was able to compile a LESS file that used the old fallback definitions.
Guess I didn't see this when I posted the change to fix it, but this is fixed upstream now.