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.
I noticed this a while back; it's due to a fault in the copy of tree.dat included with the editor. It'll be fixed with the next update; if you're wanting to use this experimental version now, here's a copy of tree.dat that's fixed.