I was using a widget to set up as a trigger and found it working and then something happened.
That is expected behaviour. If nothing is selected and you click onto a graphic style, it is applied to the page background. The same happens, if nothing is selected and you set a fill colour.
Thank you, I knew there was a simple answer. They should have talked about a sequential order in the help but as the case they usually don't explain it in laymen's terms.
Create and apply Graphic Styles in Muse