It seems that it isn't possible to upload the zip file with multiple style versions inside. I just tried Aero
and it (your extension) made ready for installation only "aero red", but unfortunately it requires the style "aero" to be installed ant it wasn't. I'm not sure if is possible solve this little problem, but it would be great, because I guess there will be more styles as this one.
EDIT: I uploaded the styles via FTP and they showed up under "Upload style". When I tried to install style via "Upload Style" I got blank page, but when I used the phpBB's "Install styles" feature I was able to install it without problem.