I fail to apply 0% opacity to the middle/content part of the section.
I have a site background set as a custom photo and both footer and header behave as expected when 0% capacity is applied. If I however add a new section, with or without pre-designed elements, I do see the background photo at the margins but the middle/content section never gets to display it (refer below snip of a section with no elements added yet).
Check whether the section has a separate Color or Pattern layer set in its Background panel, in addition to the Image opacity you’ve already adjusted — new sections can carry their own fill from the preset they were added from, and that sits independently on top of the site background. Lowering the image’s opacity doesn’t touch a color/pattern layer underneath it. If one’s set, clear it or zero its opacity too.
