How to stop old background design reappearing?

Hi

I'm revising an old course made by someone else with Presenter. The majority of the original course has a standard background - a computer monitor - on which the text is placed. I imported the course into Storyline, mocked up several background designs, removing the original background from these slides (slide by slide!) as I did this. However, I've just tried to import a quiz from another course and as soon as I click on a slide in the added scene it adds the old monitor background in and changes font colours and other colours on the slide. It also adds the old background into my mock-ups. How do I stop it from doing this?

I've just found some info on how to remove backgrounds in PowerPoint and I'm thinking perhaps it would have been better for me to have removed the backgrounds before importing the course into Storyline, however I have already edited a substantial amount of text, so the fix would then get a little more complex.

I don't know if master slides are a cause here. Though I've looked at some tutorials, I haven't created any yet, so they haven't fully sunk in. I'm also fairly new to using the 'design background' feature.

Hope you can help.

Jacinta