Where can I download the original default theme of mybb?
It's the default theme, so it's in the download of mybb!
Which folder can I find the stylesheet for it?
Hmmm...well, if you want the actual .xml file, you can always go into ACP and then scroll down to Themes and select Download. And then download the Mybb Master style theme. That should be what you want? Then I guess you can change it to suit your needs..Just make sure to import it with a different name when you're done with it!!