Perhaps .lsz isn't a registered file extension?

If so I think you could reinstall litestep using an installer, running with admin privileges for the .lsz to register. I don't know if this is right, as I didn't.

I don't use the theme installer. I rename theme .lsz's to .zip's and extract them myself, inspecting them for malpractices such as changing wallpaper or setting system colors without my permission :)