How to change themes from the front end?

I’ve been trying to create some WP themes and set up a demo blogs using those themes. Ideally, I hope I can use one blog to showcase multiple themes (as opposed to set up 3 blogs to showcase 3 different themes)

I found https://topwpthemes.com was able to do exactly what I want to accomplish. The URL https://wordpress.topwpthemes.com/index.php?wptheme=Elvira will open the same blog with the theme called Elvira.

Does anybody know how they did it? Do they use any plug-in?

Thank in advance!