I've installed Elementary Odin wanting to give the Elementary OS dark mode a try. I really liked the dark theme but was upset that when I installed a GTK application from APT not Flatpak it gives you a default Elementary light theme. I understand about Flatpak not following the system theme but why not GTK why only for Elementary applications only.
I'm not sure if it's a bug and since you'll think if you installed a GTK application from APT will use the system theme. I've found a workaround and installed Pantheon Tweaks (the new Elementary Tweaks they changed the name to not get it confused with the devs at Elementary) and turn on "Force to use dark stylesheet" and it fixed the GTK theme and so far it's using the Elementary Dark theme or close to it. I'm using XFCE's Thunar for example on the theme issue here are the screenshots since I can't upload more than 1 screenshots: https://imgur.com/a/HjMFZOE.
So I hope someone in the Elementary Dev will see this and make an update fixing the GTK issue and possiblity fix the Flatpak theme to match with Elementary theme dark and light.
Thank you all and I hope this helps :)