dpkg error: contains ununderstood data member

The .deb file is created because you told Theos build system to do that. The package install rule of the Makefile is creating the Debian package using xz compression. Now, this kind of compression is supported by versions of dpkg equal or higher than 1.15.6.

So, in order to solve your problem, you should update dpkg to a newer version or install Theos without packaging support. Probably a simple make install will do it.

In case that updating dpkg isn't possible and you don't want to install the program without package management support, the other (more painful) method is to change the algorithm in which the package is compressed. Here you have good information about how to do this.

I found out how to fix it. In $THEOS/makefiles/package/deb.mk replace this line:

 $(ECHO_NOTHING)COPYFILE_DISABLE=1 $(FAKEROOT) -r dpkg-deb -b "$(THEOS_STAGING_DIR)" "$(_THEOS_DEB_PACKAGE_FILENAME)" $(STDERR_NULL_REDIRECT)$(ECHO_END)

with this line:

$(ECHO_NOTHING)COPYFILE_DISABLE=1 $(FAKEROOT) -r dpkg-deb -Zgzip -b "$(THEOS_STAGING_DIR)" "$(_THEOS_DEB_PACKAGE_FILENAME)" $(STDERR_NULL_REDIRECT)$(ECHO_END)