Does font style and size changing of whole wordpress affect SEO or not

Better asked in Google’s forums, but generally it’s if you try to make text disappear from the webpage but still be visible for search engines that you’ll get penalized.

You will get penalized ? What does that mean ?

See https://support.google.com/webmasters/bin/answer.py?hl=en&answer=66353

Anything from your site being moved much further down the search engine results to being completely blacklisted (ie removed from Google’s index).

I think you people get it wrong.. I do not want to hide text. I just want to change the font size and style of my wordpress posts….

Try asking in Google’s forums.