I experienced a few issues with this. Hope this feedback helps:
- When changing the grid size by dragging on the screen the grid can disappear completely and it takes a little bit of dragging randomly to get it to appear again. The grid size shown on screen was bigger than 1 x 1 when it disappeared so it wasn't due to it being that small.
- It took about 3-4 seconds to switch between the colour options under the mono button.
- Once you've pressed icon and placed the box you can't undo or remove it, so if you didn't place it where you wanted you have to close the app and re-open.
- I found one sprite-sheet it won't load. I don't own this image, I was looking for sprite sheets on google to test your app with. Attaching in case it helps you debug and find the problem. I know the layout wouldn't fit your grid system.
I should add that I was running your app via Parallels on a Mac as I don't have access to a Windows PC