Rosenstein was nominated to his position by Trump himself. Mr. Mueller is a Republican, as is Deputy Attorney General Rosenstein who appointed him. Based on voter registration, 13 of the 17 attorneys working on the investigation are or have previously registered as Democrats.
That does not mean they are 'Democrats' in the sense that they work in a formal capacity as Democrats. Their expertise and work ethics -caused to be in a certain position or condition, becoming appointed for the investigation. They are attorneys involved in an investigation that in their private lives registered at some point to vote as Democrats in elections. The '13 Angry Democrats' designation is part of the President's characterization of the investigation as biased and part of some Deep State effort to remove him from office. The Special Counsel has shown that it has gone through great lengths to avoid bias, and FBI procedure to remove political bias from their work is quite thorough.