GTK applications ignoring theme when run from command-line

running NixOS, GTK applications seem to ignore my GTK theme when run from the command-line, whereas when run from their .desktop entries my applied theme appears to work as intended.
is this a known issue?

for what it’s worth, i’m using flakes, home-manager, and nixpkgs unstable.

How did you set the GTK theme and what desktop environment do you use?

You might also want to check the discussion in Setting Nautiilus GTK Theme - #2 by jtojnar

@jtojnar i set the theme from home-manager, setting gtk.theme, xdg.configFile."gtk-3.0/gtk.css" and xdg.configFile."gtk-4.0/gtk.css" (thru stylix, technically overriding gtk.theme myself by lib.mkForce).
using home-manager i also set programs.zsh.sessionVariables.GTK_THEME = "Catppuccin-Frappe-Compact-Maroon-Dark";.
as per the other thread i now also tried home.sessionVariables.GTK_THEME = "Catppuccin-Mocha-Compact-Maroon-Dark";.
i use any of these to set dark themes, whereas the unthemed GTK windows opened by command-line use the default light theme (seemingly Adwaita).

my desktop environment is niri.

edit: for posterity i’m under the impression this was a problem about xdg-desktop-portal-gnome