From my favorite theme site:
Regarding ShapeShifter, Jason Harris has officially qualified it for Mac OS 10.3.4. ShapeShifter will check online when you first apply a theme after the system update, but will eventually stop asking.
This is from Jason, sitting at Dallas Airport ready to leave for his vacation:
Whenever the OS is updated and you reboot afterwards, SS sees that it's been updated and disables the theme. Then, when you apply the theme again, SS sees that it hasn't been certified for that OS version. So it hits the Unsanity server to see if it can be certified. So far, all we've needed to do is say "it'll work fine", but there's a mechanism in place for if it needs deeper changes.