I've heard different answers to this question. Conducting a poll is obviously a hard task to pull of in an authoritarian country, considering the censorship by the state.
A quick search got me to this poll: Article, Archive
So the answer would be that only 15% of the population are pro-theocracy.
In response to the question "Islamic Republic: Yes or No?” 81% of respondents inside the country responded “No” to the Islamic Republic, 15% responded “Yes,” and 4% were not sure.