No need to repack until you want to share (and even then it's not mandatory). While tossing around with a theme it's much more convenient to have it unpacked.
By unpacking the ReaperThemeZip you actually create an independent theme (a *.ReaperTheme file and an image folder with the same name). Be sure that's the one you are pointing at in the preferences. I believe in case of the default the name would be "Default 4 unpacked" or something like that.
When you're done with messing around (which in my case is never
) you can pack the file and the image folder with any packer application that does zip. Zip them and then rename the file extension from .zip to .ReaperThemeZip is all there is to it.