When switching window themes in an Xfce desktop from the cli using: $ xfconf-query -c xsettings -p /Net/ThemeName -s <some theme> (example themes would be Blackbird and Arc - but works for any two different themes) on Debian buster (MX19), claws-mail crashes. For some reason, Debian doesn't have claws-mail-dbgsym in buster-debug repos, so I can't send a backtrace. Hopefully, it should be easy to replicate from this description.
I've seen this crash too, as have many others using a variety of different software. It is a crash in glib's g_type_check_instance_is_a. If you search for it you'll find there is a difference of opinion: a bug in GTK, a bug in the GTK theme, or a bug in GLib. I would hazard a guess at the latter ...or the former. For a pertinent example see https://gitlab.gnome.org/GNOME/glib/issues/1848 It seems that all we can do as users right now is to quit our GTK programs before changing theme.