Moving the Theme-Files

Hi everyone,
is there any goog tutorial on how to move a theme-folder to any other destination? Or can you give me tipps on how to do that? I found a lot of interesting stuff on how to do it but I am not quite sure if I will do it right.
For my purpose I have 5 Blogs which I want to use with one WordPress Installation to minimize webspace and time for upgrading each of them. Also, I would like to have the themes for them in a different folder on my server to change the https://mydomain.com/wp-content/themes/mytheme/style.css into something nicer like https://mydomain.com/style.css

I’m not really sure how to do this, allthough as mentioned above, I found some instructions in the web…