Since upgrading to 20.04 (this was fine in previous versions, 18.04 and 19.10 at least, on same hardware), when launching an app such as Synaptic, the Authentication window (from pkexec and /usr/lib/policykit-1/polkit-agent-helper-1) is not very readable in dark themes.
E.g. in Ambiant-MATE-Dark:
In Ambiant-MATE it looks OK, but I prefer to use the Dark theme!:
I haven't seen anyone else mention this so it may be a config problem on my system, but I haven't been able to fix this. Any ideas?