Slide background colour changing on Mac

So my client is having some problems viewing courses on his Mac and iOS devices.  For some reason the slide background colour is changing from white to black and for the life of me I can not figure out why.

It displays fine on Windows Chrome and Edge with a white background as it should do.  I am just waiting to hear back from my client as to whether it is localised to Safari or whether it affects Chrome as well on his Mac.

Has anyone come across anything like this before?

Below are the screenshots he sent me.  The black areas should be white.  I have checked the project file and all the slide backgrounds are set to #FFFFFF with no transparency.  The purple and white bars at the top are shapes that sit on the slide master and don't seem to be affected.  The player is also on the Light setting so the page background is displaying white as it should do.

The 3rd image is a screenshot from my machine that shows what it should look like.

Edit: I have now managed to test this myself on a Mac and it seems localised to Safari.  Tried it on Chrome and it was fine.  Nothing to do with Dark Mode either as I tried it with Dark Mode turned on and off.