If you have a dark app, say spotify, that uses a light titlebar for some reason, and want it to use a dark titlebar, look no further! This script launches spotify and changes it's titlebar to dark:

#!/bin/bash
spotify &
sleep 1
xprop -f _GTK_THEME_VARIANT 8u -set _GTK_THEME_VARIANT "dark" -name "Spotify"

Save it somewhere, and change the .desktop file (/usr/share/applications/spotify.desktop) so it points to the saved script (a.k.a, the "Exec" value)

The sleep is needed because the xprop command only set's the theme of visible windows, hence the sleep, so that spotify has a second to startup.

As I said, this can be applied to other apps as well, just change out "Spotify" for the name of the app