Note that I discovered this before namida posted the recent version of the experimental editor that displays backgrounds while editing levels.
For some reason, while playtesting my Tree levels, it shows a brown background.
This only occurs when I open Test Mode with the experimental version of the editor. If I open the same level from the NeoLemmix.EXE, it shows the normal black background. When checking Tree levels in LPII/LPO, the background is also black.