It's really a shame that this nice-looking and widespread theme didn't receive the required updates for such a long time. I read bug #219254 and bug #226469, and I didn't understand why such simple changes have not been applied yet. If the reason is about accountability... well, I'm not a developer, but I can take the commitment to update it from upstream by submitting patches as PR from now on.
Please find the attached patch, which is working for me to update to the latest version.
A commit references this bug:
Date: Sun Jan 27 13:33:34 UTC 2019
New revision: 491349
x11-themes/greybird-theme: update to 3.22.9
Submitted by: Andrew <email@example.com>