Change Floor Tiles
Complete all 16 practice rounds (8 1UP/CPU and 8 1UP). The background
color of the main screen will now change to red. Go to the options
screen and you'll see a new option called floor. This will let you
choose from many different floor sets such as the Mona Lisa and the
American Flag.
Change Puzzle Pieces
To change the tedrads from T and L shaped pieces to triangles, short
bars and large squares, first complete all practice rounds. You will
then have a red background. Then get an "OK" rating in all modes except practice and multiplay. (When you get an "OK" rating, game icons will flash green.) Once you have achieved an "OK" rating in all required sections, your background color will change to green. Then in all modes of play except practice, you'll see a little blue guy. He represents the default pieces. Click on him and you'll see a duck. This represents the new set.
Credit For Codes Goes To: IGN 64