When clicking on a theme name in the tree view, nothing happens. For Stephan Arts, it results in a crash. Relaunching xfwm4 or xfsettingsd brings the new theme. This happens with the current git master.
As far as I am concerned the code for xfwm4 has not changed so I don't see how this could be a bug introduced in xfwm4. Beside, I am not using current git HEAD (and don't plan to do so any time soon) so I have no way to investigate this issue with so little data. If you want me to help, please provide supporting data (xsession-errors would be a start, a core file better and a gdb backtrace a must). Help me help you... or else patch welcome.
As you said on IRC, this had nothing to do with xxfwm4, it was an xfconf problem which is now fixed. *** This bug has been marked as a duplicate of bug 5734 ***