Some applications like gnome-tweaks, gnome-terminal, etc are not using the latest lib-adwaita theme.
Am a beginner on nixos, so is there anything I need to do here to enable or install it?
Those apps have not yet been ported to libadwaita so it is working as expected.
If you really want them to use the latest Adwaita theme now, you could try installing adw-gtk3 GTK theme,which tries to replicate the look. Just keep in mind that the theme is not an official GNOME project and it is not supported by GNOME.
oh, thanks for the information mate. it works