Open new issue
And thanks for the bug report. I installed fluxbox on my machine to give it a try. Now xplanetFX is able a) to work with the fluxbox autostart system and b) to change backgrounds in fluxbox with its native tools (feh or fbsetbg). So your desktop environment should be supported OOTB now.
brew install xplanetfx
The following dependency will be installed if you install Xplanetfx:X11.