Dark Mode!

Changelog:

• Built with the new LibNx update so controls will work fine in 9.0.0.
• Added dark mode, enabled by default (press "-" to toggle).
  • Due to me just inverting colors, colored images wouldn't look so nice.
• When reading a book there is a help menu if you press the "+" button.
• The book folder is created by default on the start.

Things to look forward to:

• Other file type support

Enjoy!