Question about using many Themezee themes in 1 blog

Hello,
Sorry for my bad english (I’m french).
I use Courage theme (free version) since 2015 in my webzine site : https://www.rythmes-croises.org/
I wonder if I could install a few themes and allow the visitor to choose between these themes?
Or do I have to chance all settings before I want to change the theme of the site?
When I try to previsualize a different theme, the header image is staying after the menu and I can’t take it off. There is no image in the header image place of the theme previsualisation.
I use the events calendar but in some themes the characters are much bigger in the side bar. How to change the characters size for 1 blog?
Is it the same with the Pro version for all these questions?
Thanks in advance for your answers.

The page I need help with: [log in to see the link]