I mean in the very material (CoronaPhysicalMtl or CoronaLegacyMtl) itself:
Base level 1.0, base color white, refraction 1.0, roughness 0.0, thin shell enabled.
Just tested this and it makes zero difference. The only thing that makes a difference is the IOR.
Base Level 1, Base Colour White, Refraction 1, Roughness 0, Thin Shell, IOR 1.52 = darker scene
Base Level 0, Base Colour White, Refraction 1, Roughness 0, Thin Shell, IOR 1.52 = same as previous
Base Level 1, Base Colour White, Refraction 1, Roughness 0, Thin Shell, IOR 1.00 = brighter scene
Base Level 0, Base Colour White, Refraction 1, Roughness 0, Thin Shell, IOR 1.00 = same as previous