[aplaice]

Unfortunately, GTK 4 has removed key themes. Eventually, Firefox, Chrom(e|ium) and all actual GTK applications will migrate to GTK 4, so this is not very future-proof.

I use a variant of the emacs key theme, so I'd be happy if there were a workaround...

[tmtvl]

It's unfortunate that the GTK devs feel they have to remove features which their users find useful to keep GTK maintainable. Keeping software complexity under control is a difficult task, and I can't begrudge them their choice.