Question: Which themes have had corresponding exhibitions that have had attendance both below 100 and above 500?
SQL: SELECT T2.theme FROM exhibition_record AS T1 JOIN exhibition AS T2 ON T1.exhibition_id  =  T2.exhibition_id WHERE T1.attendance  <  100 INTERSECT SELECT T2.theme FROM exhibition_record AS T1 JOIN exhibition AS T2 ON T1.exhibition_id  =  T2.exhibition_id WHERE T1.attendance  >  500