Powerpoint, change background

I don't think so. Background images are defined by the master slide in Powerpoint, and FME doesn't touch those. If you create a new powerpoint with the new style, then you can create new slides, but can't amend existing ones. You'd have to save the original powerpoint as an FME-readable format (PDF?!), read it back with FME, reconstruct/restyle it, then write it to a powerpoint using the new template.

But I think you'd have to be working with a large number of powerpoints, to spend time getting that to work. Otherwise you may as just well open each in turn and set the background manually.

I would investigate using python-pptx.

I've had very good success with the corresponding python-docx package.

Alternatively you could change the extension from pptx to zip, and then use the xml readers/writers and transformers to update the slide.