If your GTK/Qt applications installed with Nix isn’t using your system’s specified font, you can try running them with the FONTCONFIG_PATH environment variable.

Example:

FONTCONFIG_PATH=~/.guix-profile/etc/fonts/ liferea

If it works, you may add the environment variable permanently to your home profile along with other Nix-related variables:

(simple-service 'setup-nix-env-vars
		home-environment-variables-service-type
		`(("XDG_DATA_DIRS" . "$XDG_DATA_DIRS:$HOME/.nix-profile/share")
	      ("NIXPKGS_ALLOW_UNFREE" . "1")
	      ("FONTCONFIG_PATH=$HOME/.guix-profile/etc/fonts")))

This is for people who didn’t bother reading the console logs :)

Reference article: Fontconfig error: cannot load default config file