I want to change the master slide while running the course.
I want the user to be able to see during the course, to change the background color
please help

You can't really change the master slide itself but you can more easily just add a different colour layer as a background.
Just add a gradient object, change the colour as required then add the content on top of that.

Simple

i want user when the cource is running can change the backgrond color

thank you so much