I've installed a new selfmade style and played around a bit. Then i've tried to uninstall it, i modified it and installed it a second time.
I realised that the newly installed style gets a new id but no new datarows are installed for the imageset/template/theme because the old ones (from the deleted one) was still present with the same name! Though the new style got the old data assigned and thats not what a user expects.
A user who installes a new style with the same name of a style that was previously installed expects that the theme data of the new style is used. But there is still old the theme in the database present (e.g. the parsed css source).