What styles of music does the synthesizer play?
The person or persons who program and play the synthesizer determine the style. the synthesizer in itself cannot play, it is just a machine. Just as you can play chamber music or jazz on a violin, you can play different styles on a synthesizer.
Is it possible to play Genesis by Justice with instruments only like an instrumental version but still sounds good?
The standard fare for an orchestra to play is Classical music. However, the orchestra is not limited to just playing Classical as they can and do play many other genre's of music. Take for instance the Glenn Miller Orchestra - yes, still and "orchestra", and yet they play mostly music from the Swing era. The term "orchestra" can be applied to many different groups and as many different styles of music.
Stevie's music has spanned a wide variety of aspects. In the beginning, it was the Motown sound. In the '70s, he developed new influential styles of vocalization in pop/R&B and funky keyboard sounds (synthesizer). Most of his hit songs are up-tempo and uplifting. Many of his lyrics deal with social and spiritual issues, as well as romantic love.