In a new Manjaro Openbox installation, the file /etc/skel/.config/gtk-3.0/gtk.css starts with the following two lines:
By default, this file gets copied to the home directory of each user, of course.
When you then try to run gvim from a shell prompt, you get the following error message:
(gvim:): Gtk-WARNING **: 00:25:41.848: Theme parsing error: gtk.css:2:19: 'solid' is not a valid color name
The documentation for the box-shadow property certainly doesn't look as though it would allow this value. I can't work out the intention of whoever put this in - did they want to create a drop shadow, or did they mean to set the "border" property, or something else altogether?
I've commented out that property setting in all instances of this file on my system, and gvim now starts without the error. That's fine for my purposes. But this ought to be corrected for the benefit of future Manjaro users. I can't see how to raise a bug report other than by posting here. I've found https://gitlab.manjaro.org/profiles-and-settings/manjaro-theme-settings/blob/master/skel/.config/gtk-3.0/gtk.css, but that seems to have a different version of this file. I guess the repository for Manjaro Openbox lives somewhere else. I'd be happy to help fix this if I were pointed in the right direction...
Best wishes and thanks to everyone involved in Manjaro.