How can I create my own themes for diashows?

Hi together,

after I created a stunning slideshow with PSE 8 in 2010, I was a bit disappointed, when I updated to PSE 18 (2020) where Themes were already created and only 5 of them available.

I my slideshow with PSE 8 I could everything do by myself, chose the backround, decide the way, how the slides enter the screen, how many are on the screen at the same time, in which direction the zoom should run, could exactly cut the part of the music I wanted to be played on a special own music lane at the bottom, and something more.

How can I do that now?

I only find on 4 minute tutorial, which describes the quick and dirty method, but nothing more.

Is it no more possible, or why is so complicate to find?

Would appreciate any help.

Regards Gepho