。
Both perspectives share a common theme.
Literal
Both [possessive-の] perspectives [in-には] certain common [possessive-の] theme [subject-が] exists.
ある before 共通のテーマ functions as a pre-noun adjective (連体詞) meaning 'a certain' — distinct from the existential verb ある at the end. This double ある in one sentence is a structural curiosity. 展望 (perspective/outlook) and 共通 (common/shared) are formal written vocabulary.