Exactly, if you remove the /site/accounts folder, your user data will be lost and you will have to recreate it. Usually no big deal, and you won’t loose anything, you just have to create a new user, but if you have a lot of users, that might easily be annoying. You can move the accounts folder to another location to prevent accidental overwriting in the future.
But you should keep in mind that you can’t really change “themes” with Kirby like you are maybe used to from Wordpress or the like. A “theme” in Kirby is in most cases not a plugin but a collection of templates, snippets, stylesheets, content files and more, that is not usually easy to combine with other templates, snippets etc from other themes. Your content structure, naming of files and templates must all fit together.
Have you seen the Getting Started guide? Maybe that will give you a better understanding of how Kirby works.