There is no doubt that themes can effect search engine rankings but on a very low level. THe most likely way is their load speed. Some of the big fancy pro themes out there look good but there is so much going on that it takes forever to load and remember the majority of visitors might not have lightning fast broadband.
If your blog takes a long time to load your bounce rate is going to sky rocket because most people are not going to sit there and wait for your site.
High bounce rate means google is going to penalise you so anyone saying themes do not have any effect, you're just plain wrong