Now. Here is how you do it. You can remove a lot of XFCE4 bloat as well because you don't need most of xfce to accomplish this just a few parts.

cd /etc/xdg/openbox
nano autostart 

Go to the bottom and add your start programs here are some examples

xfce4-panel &
xfsettingsd & 
compmgr -f

If you want you can add other start up programs, since Openbox has no desktop wallpaper. I included nitrogen to boot as well so I can choose my background at login. Or

 xfdesktop &
tint2 &
compmgr -f

ext. You can customize as you like

You can remove a lot of Xfce4, I did this from a minimal install I have less than 1000 packages with a fully functioning DE. Funny thing is it will break log out in the applications menu, because you are not using an XFCE session. However, from a minimal install the log out option is absent from my applications menu which really just is the icing on the cake.

side note, you will need gtk2 engine components and gir1 packages for the theming to look similar to xfce with openbox as the WM. Your appearance settings for xfce will work after including xfsettingsd in the auto start. Although using xfdesktop will make openbox right clicking not work anywhere untill you kill it.

If any one can find out how to use open box right click on xfdesktop it would be a great combo.

I might of mis-read the OP. Your Xfce theme will have to match. I am using Adwita and Dusk i believe but I did go through the gtk-2.0 and gtk-3.0 to modify the colours to how I liked them. Openbox I am using Breeze with custom menu colours.

Inshort. Themes can be found in /usr/share/themes.
XFCE elements including thunar and various applications will use GTK theming. Inside your themes folder you will need a theme with both openbox themes and gtk-2.0 and gtk-3.0 themes if you wish to use openbox as your windows manager and retain some of your xfce4 theming.
It can be as simple as moving your nightmare theme into your xfce4 theme folder and combining the two. The combinations are endless and gtk-2.0 and openbox colour schemes are very simple to edit with simple hex value changes using your text editor of choice.  gtk-3.0 can be the same process but requires a bit more digging as they are a bit more complex.
Hex values will start with a #a522a5 where colours are divided into two. R, G, and B. With F being the highest value and #FFFFFF being white and #000000 being black as it gets. I'm sure you guys will find it really simple. Have fun,

You can test it like so:

openbox --replace

If that works, you can run an xfce session with openbox and any other gtk theme or settings you want.
(Nightmare goes well with the albatross theme, btw.)

@FSR: It's in Miyolinux, that's where I got it too.

Hello

Is it possible to implement the openbox Nightmare theme in XFCE? https://www.google.fr/search?q=nightmar … HVHC1qukM:

It comes with the Openbox configuration manager in Openbox, LXDE and LXQT but I have never been able to install it in other DE. 

Thanks

Xfce doesn't understand openbox theming.  I know because I adapted the Clearlooks Phenix Purpy color theme for openbox.  Completely different animal.  You would have to find a theme that works with Xfce and reverse engineer the colors.  To get the layout like that be prepared to drill down deeply into the theme structure.  I dipped a toe there and it was more than I wanted to take on.

Is it possible to implement the openbox Nightmare theme in XFCE? https://www.google.fr/search?q=nightmar … HVHC1qukM:

It comes with the Openbox configuration manager in Openbox, LXDE and LXQT but I have never been able to install it in other DE. 

Thanks