I saw this on Flickr and just had to comment. As many of you know, I have written themes for OSX. And, I can tell you how this was most probably done:
The file Extras.rsrc still kinda works in the betas for 10.5 although, I hope that it will be removed before release.
Anyway, Installer based themes (though out of favor in 10.4) still work by replacing Extras.rsrc among other files.
Someone swapped out the normal Extras.rsrc and took some screen shots. Since most of the guts of 10.5 do not depend upon Extras.rsrc anymore, I am confident that 10.5 would run with a 10.4 Extras file for now.
Note that you do not see any signs of resolution independence! That is what would break this fake theme (since res independence calls to images not stored in Extras.rsrc).