iMovie 09 (v8.0) has 20 built-in transitions e.g. Cross Dissolve, Cross Blur, etc.
However, it only has five Themes: Photo Album, Bulletin Board, Comic Book, Scrapbook, and Filmstrip.
Either way, I think you're limited to what it has. The current iMovie does not support plug-ins the way iMovie 6 and HD did. Hopefully there will be an iMovie 10 with expanded Themes.