How to Customize Windows 10 Themes (wish I knew)

It used to be very easy in Windows 7 to modify and customize existing Themes. We had four functions at the bottom of the Theme selection page to modify "Desktop Background", "Window Color", "Sounds", and "Screen Saver". Well, it seems that some bone head at M$ has decided that we can't have this something that works anymore! Yes, some of the functionality has come forward to W10 although it now seems to be a total kludge to get to it these days. For example, the only way I've found to get to adjust the Screen Saver functionality is to perform a search on it. It does not seem to be on any menu that I can find. As for "Desktop Background" and "Window Color"; I've yet to find this functionality in W10. So why in the world did M$ eliminate this stuff?

And before somebody exclaims that you can modify the background in W10... Please explain how you add a photo to a Theme group these days. For extra credit, tell me what menu you can use to access this functionality from.