Er...why do that? Just set the option in folder options to see extensions, backup classic.jar, rename the .jar to .zip, open it, mess around, then rename it back. That's what I do.
I open (and keep open) the .jar archive with WinRAR and extract the contents to a folder and edit whatever tickles my fancy. Then I navigate to the original copy of the file I edited in WinRAR and drag the edited version in from the other folder to replace it. Then WinRAR will repackage it for you. Drag the repackaged .jar to the Themes window et voilĂ .
what packer you use? I just did that with thunderbirds classic.jar using 7zip and no probs at all, when I was still using winrar it worked with that too so...dunno
The best way is to reopen the jar file you extracted (with winrar) and browse to the location of the files you want to replace, now you can just drag the edited ones back in, just hit ok and voilĂ !