is there any realtionship between themes or templates and pr? Frequent changes in themes of mysite will affect pr?
No, Pr is seperate. Frequent changes to your site will not effect your Pr. You get Pr from links to your site from other sites. Changes to your site might affect how you rank and changes to your site architecture might change the PR of individual pages, but It will not affect your site PR
I don't think there is any relationship between Themes, Templates and PR. To improve PR, build back links.
There is no relationship between the themes, templates and page rank however, sometimes if you change the programming language of your site this can temporarily change your page rank.
As the name suggests, Page Rank is related to your page, not your site/domain. I don't see any relationship of changing template with page rank, unless you change your site structure. Changing site structure will surely affect the page rank.