A previous version theme is usually easy to update for 1.5, so they are not terribly "version specific". The biggest thing (I think) is that if you want to use a widget zone instead of coding individual widgets into a sidebar, or whatever,
you edit the theme's site.master file to add this tag:
Instead of invidividual tags (which will be in the older themes) to get that functionality.
A nice listing of the various tags you might want to add or modify in the theme is here:
Themes are scattered hither an yon across the Net and are fairly easy to find with a google search.