I use the vertex theme,(not vertex dark), in XFCE 3.2.1 and because i am not relaxed like a sniper
it is almost impossible to get the 1px window grip at 1st attempt.
Especially the one to adjust the window size,horizontal and vertical, at the same time.
If you're "tremulous" like me, the solution / improvement is to install the "axe" window frame theme.
You get rounded corners and a slightly wider edge and easy to get border grips.
If you want to have it too, go to:
https://www.xfce-look.org/content/show. ... tent=73291
Download the 73291-axe.tar.gz,
extract it and copy the "axe rounded" folder into /usr/share/themes/
(settings > Window Manager and choose the new axe rounded theme)
This is how it looks like:

8)