Well, there is one thing I would hesitate to think about changing page rankings and themes. It would be the way the page is processed.
Some themes, when you look at the html, you see that they processed the content first... then maybe the sidebar on the right. others you might see the sidebar on the right, then the sidebar on the left, and then the content of the page processed in that order. Different theme process the html in different orderes. While that stand alone may not change page ranking, you also have to think about how different search engines will accept data. Some may only take X KB before they "rank". That is why it is best to choose a theme that actually processes the content of the post first then the sidebars. the data is closer to the top. Sounds strange I know, but it is true.. and you can goto differnt sites and view source to see different processing of differrnt themes.
Also, soemthing else to consider .. if you have worked with theme #1 for example.. for a year and it is tweaked jsut how you want it. for seo, search engines, etc... and all the sudden you switch to theme #2.. it is not tweaked. So, google could be in the middle of an update (it is still in the middle of one now from last week because mine goes from PR 5 to 6 on and off depending on the centers that are pulling the rank and updating.. page rank is determined by the values in mulitple data centers from google) so... if yours all the sudden is not tweaked and it is doing an update...
and we all know how some themes are structured W3c valid and some arent,. once again if you switch and it is all not 100% valid as it is being cached or checked or tabulated for rank at that moment.. say i change to a new theme and it takes me 2 weeks to make it 100% w3c compliant... you never know when it is doing its thing.
IF it happens to get updated before it is W3C complaint, if there are a bunch of errors, it COULD lower a page rank.
Also, if you change themes from one that is less SEO tweaked to one that is more SEO tweaked, or one that has better HTML structure, less errors, etc, it can even help..
True, this will not be ALL PR is based on, but yes, it *can* be a factor in page rank.
Sorry to make this so long and boring.. in a sum, no, google doesnt have eyes, it jsut reads the code.. just make sure the theme switched to has as good code as your present one validation and seo wise.. or even better ;)