Which themes have had corresponding exhibitions that have had attendance both below 100 and above 500?
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