GTK theme while running emacs without gnome shell

[Kenned Doll]

I built emacs 28.1 from source on ubuntu 22.04 running under WSL2 on 
windows 11. I have the theme specified using the GTK_THEME environment 
variable. The theme works as expected in gnome-terminal and nautilus, 
but for emacs it only works within the window. The border and title bar 
are white and I'm using Adwaita-dark theme.

So, first, do you know how I can get the theme to apply to the title bar 
and border?

Also, I thought the titlebar and border were ordinarily supplied by a 
desktop manager or desktop shell. Since I'm not running a DM, where is 
that coming from?