Changing monospace fonts system-wide

Add this to yours ~/.config/fontconfig/fonts.conf file to set Font B as default monospace font

 <match target="pattern">
  <test name="family" qual="any">
   <string>monospace</string>
  </test>
  <edit binding="strong" mode="prepend" name="family">
   <string>Font B</string>
  </edit>
 </match>

Or to /etc/fonts/local.conf to set it system-wide

I found this to be working:

gsettings set org.gnome.desktop.interface monospace-font-name 'Noto Mono 11' (Probably not all apps, gedit changes instantly)