Where is fastfetch retrieving Theme, Icon, font, cursor info from?

[tuvin5f01]

Currently I am using NixOS and home-manager.
And I am trying to figure out how can I change themes declaratively.
I set the session variables GTK_THEME, ICON_THEME, XCURSOR_THEME, XCURSOR_SIZE to a proper value.
It works mostly but I realized fastfetch says I chose Adwaita(default) as my gtk2, gtk4 theme. While gtk3 theme is correct.

I would like to know where this program reads to get gtk2 and gtk4 info, in order to solve my problem.

Thanks in advance.

[CarterLi]

It's complex. You can read the source code: https://github.com/fastfetch-cli/fastfetch/blob/dev/src/detection/gtk_qt/gtk.c#L37-L152