Complex sentence: It was a theatre of painters, for whom the visual aspect of a performance (such as costumes, stage design, actors' movements) played a definitely crucial role, which was significant of Post-Impressionist influence (mainly of the Kapists).
Split into simpler sentences: It was a theatre of painters, for whom the visual aspect of a performance (such as costumes, stage design, actors' movements) played a decidedly crucial role. The aesthetics of the theatre was significant of Post-Impressionist influence (mainly of the Kapists).