The first image has a white background, so it is fine. But your app's output doesn't set the background colour, so it is transparent and shows up as black on dark grey in night mode. Hard to see the output result on the github page in night mode as a result. Some image viewers will show that output with a checkerboard background to reflect that its colour has not been set to white.
This reminds me a little of the idea behind guitar tab. It should be pretty easy for people to use. I do think the limitation to naturals and sharps will make the actual scale too ambiguous, but I get that you're trying to save visual space.