The question is "Identify the thematic relations associated with each DP or embedded CP..."
"It is raining in San Francisco".
I believe "in San Francisco" is "location".
Then what is "It"?! Is it agent? experiencer? nothings?!
In your example, it is used as a pure dummy it (sometimes called empty it) without a related postponed subject without a verb and adjective phrase denoting weather condition.It is also used to denote distance and time. (It is ten o'clock or It is two miles ahead.)It simply fulfils the subject requirement in English.