I had a tree that was covering up the view of my house. Instead of hiding the tree entirely, I wanted to lower the opacity so that I could see the house more clearly.
First I went in the Visibility/Graphic Overrides window for the view, found the plantings category and set the Transparency of Projection/Surface to 80% or so. That made my tree transparent in one view. But I had other views and I didn’t want to do that for all of them.
So I cleared that graphic override and instead went to object styles. Choosing plantings again, I assigned a default material that was 80% percent transparent. Now my tree is properly see-through in all my views.