Arc-Dark based theme inspired by OSX for Openbox. It's mainly based on this theme, but with a dark style
In this image, I'm showing nemo while using this openbox theme with Arc-Dark GTK theme.
You should be able to install this theme as any openbox theme: Move folder(in this case, Arc-Dark-OSX) to /usr/share/themes. Then use lxappearance, obconf or similar to set the theme.