Site background image and post/page background colour/image

Hi

As the title suggests how can I add a colour (transparent) or possible image behind post or page content but on top of the global site background image. Currently I’ve loaded a global background image. But this makes some of the text within posts/pages unreadable or hard to read.Is it possible for a transparent colour/image to be loaded that will sit behind the text but over the global site background image.
It doesn’t need to be post/page specific this will also be a global setting/adjustment.

Best way to explain is via an image