I cannot copy the background colour from one slide to the others
I can copy the style of text, etc, but the background will not copy
iMac 21.5″, macOS 15.3
You can simply copy and paste the slide with the background you want using Cmd + C and Cmd + V, then add your content on top.
Another possible way is to use slide layouts. Go to the menu bar and select View > Edit Slide Layouts. This works like a master slide. If you set the background in the layout, any new slides based on that layout will automatically have the same background.