Big change to Background Tiles. You now can have up to 10 Tile Sets, with up to 36 tile frames in each for use in your own rooms and levels. Tile Set selection has moved down into the items inventory area. The frames area on the right now scrolls to save space. There are some new buttons: 

COPY a tile or item frame
PASTE to reuse it, which should save you some work.
CLEAR will erase a frame, and if you have a color selected it will fill the frame with that color.

NOTE: only ONE tile set can be used in the overworld and other public areas. Technically you can use more, but the game will only render one of them.  (Well, actually you can use a different set for front and back layers) This is to keep the public areas from slowing down due to too many layers. The overworld might look a little messy until people go in and fix their areas and consolidate things into one set.