I've also noticed that when I add the theme in admincp and try to edit it, it displays this message:

SplFileObject::__construct(/home/equiland/public_html/templates/bookstore/bookstore/style.css): failed to open stream: No such file or directory

I have no idea then, unless you edited some code somewhere. What did you do exactly? I didn't have any problems doing it myself.

I did exactly what it said to do in the tutorial :/ I don't even know how to get it back to normal again.

To go back to the original theme, just go to phpMyAdmin, adopts_users_options, and edit your row to change the theme back to "main" (no caps). Then you can go to the adminCP and set the default theme back to main.

I've managed to get the theme back to normal! Thank you! Not sure how I'm going to add themes as I'm worried I'll mess it up again xD