None of the default themes provide borderless windows.
All of the default themes provided with xfwm4 have too much window border. There doesn't seem to be any easily selected option for borderless windows.
Attached is my preferred "Borderless" window theme. I got rid of the left and right window borders entirely and shrunk the bottom border by a few pixels. You can't resize windows on the left or right any more, now that there is no border there. But you can still resize the window using the bottom and top borders/corners.
Most applications I use have a vertical scroll bar along the right hand side. I used to unintentionally resize my window while trying to use that scroll bar. With this Borderless theme, accidentally resizing the window is far less likely to occur, and I've regained a little screen real estate too.