I am actually a big fan of the brush strokes on the tree. the colors look great. I also am a big fan of thick outlines. I use a lot of thick outlines, I find sometimes a good thing to do is not to make them all black. maybe use a dark brown for the tree base and a dark green for leave outlines etc... it really makes the design pop without looking like a coloring book. I also don't think it is a bad idea to work on several backgrounds at the same time, you can get a better overall feel of where you wanna go without spending too much time on a single piece, and then go in for more detail later. keep up the good work.