Introduction
In Xfce, the overall look of windows and applications is controlled not just by the GTK theme applied, but also by the Window Manager (WM) theme which “is responsible for the placement of windows on the screen, provides the window decorations and allows you for instance to move, resize or close them“ (Link 1)
Even though most GTK themes will include a subfolder with the accompanying WM theme, you can choose to mix and match your GTK theme with the window decorations and borders from any other WM theme.