mutter should import /etc/X11/XResources and ~/.Xresources after starting Xwayland
Currently XResource based config for old X11 apps is ignored when running a Wayland session.
I've discussed how to fix this with @ofourdan and we both agree that the importing of Xresources does not belong in Xwayland (Xorg also does not do this itself) but needs to be done by the code launching Xwayland. Which in case of a mutter based desktop is mutter.
Note: I've put looking into making this happen on my TODO list (below a bunch of other higher prio projects).