It is only possible to set default monospace font via fonts.conf file. The same may be applied for default sans and default serif fonts, but they are used rare.
Created attachment 6997 Add Default Monospace Font chooser to Appearance dialog
Implemented by https://git.xfce.org/xfce/xfce4-settings/commit/?id=3c3dc977df7b80e38edb3a2c5393f26250b0f73c
Thanks a lot!
The next step is to teach apps (e.g. xfce4-terminal) respect this setting :)
I thought this patch edits fontconfig file. Every Linux app already respects this file. E. G. selecting monospace font in mousepad results in font specified in fonts.conf
No, the patch stores the monospace font setting in xsettings database, similarly to the other font. I know that there're also font settings in dconf database which are used by gnome but xfce doesn't seem to be using that ones. I will need to look further into fontconfig usage as you suggested.
As far as I can tell, mousepad has default font hardcoded to "Monospace"; I wasn't able to find any read from any storage when selecting "Use default font". In addition to what I've already said about gnome, I doubt there's a standard non-distro-specific way of specifying default font.
Xfce would benefit if it would have stored font settings in dconf too. Most of linux apps respect it. E. G. I've reported bug to chrome about font AA (https://bugs.chromium.org/p/chromium/issues/detail?id=590831). It seems to me that despite fonst.conf it also reads dconf (cause there are no problems in gnome). On the other hand Serif fonts settings are applied to every GTK app