On Host, after exporting the libreoffice from distrobox, edit e.g. ~/.local/share/applications/abox-libreoffice-calc.desktop and add "env GTK_THEME=Adwaita:dark":
Exec=env GTK_THEME=Adwaita:dark /run/current-system/sw/bin/distrobox-enter -n abox -- libreoffice --writer %U