I am using KDE, but this issue applies to GNOME as well (just tested it).
I haven't used LibreOffice for a while, and today I started it and saw that:


Clearly, libreoffice ignores my QT theme, GTK theme, and font settings.
Previously (a couple of month ago) it was OK, "Breeze" theme from KDE worked, and fonts were applied correctly as well.
nix-shell -p libreoffice --run "libreoffice"
"x86_64-linux"Linux 4.17.6, NixOS, 18.03.132865.411cc559c05 (Impala)yesnonix-env (Nix) 2.0.4"nixos-18.03.132865.411cc559c05, nixos-18.03-18.03"/nix/var/nix/profiles/per-user/root/channels/nixos/nixpkgsExpected look:

Same here. LibreOffice always looked like that since I set up the machine with NixOS 17.09 though... On 18.03 the problem persisted.
Well, I switched from 17.03 to 18.03 without 17.09, so maybe that's the issue. I clearly remember it looking good on my machine.
I have the same problem (clean 18.03 install). Setting SAL_USE_VCLPLUGIN=kde4 as described here fixes it for me (any of gtk/gkt3/kde4 seem to work).
https://github.com/NixOS/nixpkgs/blob/95a819c0026a7a3c0dd2099e1c53fea95430c505/pkgs/applications/office/libreoffice/wrapper.sh#L3 seems to be the issue. Maybe we could add an optional thing (I don't know what they're called; something like nix-shell -p "libreoffice.override{vclPlugin=\"kde4\"}"), because right now the actual value is gen, which stands for "generic".
Actually, it seems like it's already fixed, the fix just didn't make it to release yet: https://github.com/NixOS/nixpkgs/commit/1f52bfb67cfb0c2ceec862e1aab9f5667c9100ee
cc @7c6f434c : when will this be merged into release?
Hm, I never think in terms of stable branches… Well, I doubt I will get around to it before a couple of weeks, but if you create a cherry-pick PR and mention me, I will probably merge it in a couple of days.
All good, should be fixed with #44090 . It should appear in channels in a day or two.