Reductase component of a two-component system that catalyzes the first step in the aerobic styrene degradation pathway by enantioselective epoxidation of the vinyl side chain. Utilizes NADH to reduce FAD, which is then transferred to the styrene monooxygenase StyA. a reduced flavin + NAD(+) = an oxidized flavin + 2 H(+) + NADH Aromatic compound metabolism. Homodimer. Belongs to the non-flavoprotein flavin reductase family.