set-dark-theme-variant.py relies on theme variants that themes may not have

The title bar - which used to be dark themed on this flatpak version - became light themed since the updates on 4th April. Closing and reopening doesn't help (it used to when it happened with the previous versions).
I'm using elementary os 7, switching on the system-wide dark mode doesn't help (freedesktop color scheme preference), however, forcing dark mode from "Elementary tweaks" does make it dark.