I'm not sure if it's stemming from a bug or I've just missed a package or service I should've included but I find there're several programs I have installed which aren't saving their settings.
Catfish and thunar are two easy examples while gnome-calendar, arandr, and viewnior do save their preferences, when modified, just fine. In a sort of weird middle-ground, lxappearance will save, say, an icon change (I can check .config/gtk-3.0/settings.ini and see that the icons have been updated) but, upon opening it, again, I find that it says the previous icon set is what's been chosen (this doesn't affect the settings.ini file, though; I would need to actually reselect the old icon set and hit Apply for that to get updated, once more).
In a sort of similar vein, icons can't seem to be recognized for particular programs; thunar and gnome-screenshot are easy examples and I've attached an image indicating what I mean. Catfish is fine but I think it's falling back to the HighContrast iconset (interestingly, the only iconset that seems to work when I set it in lxappearance). rofi, even, isn't able to provide any application icons when using drun mode. lxappearance and nitrogren are two I've noticed using the icons I set (Papirus-Light, mostly).
I thought it might be an issue with how XDG_DATA_DIRS may be set but it's set to both /run/current-system/profile and my .guix-profile's share directories; my .guix-profile/share/icons only has hicolor in is but /run/current-system/profile/share/icons has all the expected icons in it, including hicolor. At least, it certainly is when I open a terminal but even launching, say, thunar from the terminal still has the exact same behavior.
I'll put the services and packages of my system config below but let me know if there's anything else I could provide which would also be helpful.
(list (specification->package "openbox")
(list git "out")
(list git "credential-libsecret")
gnome-font-viewer ;; gsettings-desktop-schemas
(service slim-service-type (slim-configuration
(modify-services %desktop-services (delete gdm-service-type))))