If you want to use GTK, first change your display manager. Perform following operations.
sudo apt-get install gdm
sudo dpkg-reconfigure lightdm
You will get a screen like this...
choose "gdm" using your cursor and then press enter.
Then you can restart the machine (optional), or just log out and login to "gnome-shell"
Therafter you can use "tweak-tool"
sudo apt-get install gnome-tweak-tool
Open the tweak-tool. Here you can choose whatever theme you may like, but make sure "Window" & "GTK+" has the same theme, else you will get the issue as you mentioned, that the window headers will be different.
I personally like Vertex theme. Try it out.. its quite brilliant,especially the dark one.