The API could be better for this but it is possible. The thing in the browser that you select to set if it's translucent or not is a WorkSurface object. What's missing in the API is the ability to get to the work surface from either the base feature or the body that was created. If you're doing the import of the STEP then you can know that the work surface just created will be the last one. The code below takes advantage of that.
Public Sub SetOpaque()
Dim partDoc As PartDocument
Set partDoc = ThisApplication.ActiveDocument
Dim comp As PartComponentDefinition
Set comp = partDoc.ComponentDefinition
Dim workSurf As WorkSurface
Set workSurf = comp.WorkSurfaces.Item(comp.WorkSurfaces.Count)
workSurf.Translucent = False
End Sub
---------------------------------------------------------------
Brian EkinsInventor and Fusion 360 API Expert
Website/Blog:
https://EkinsSolutions.com