Did you try putting more structure (concentric outlines or a gradient or something) in the input image? Seems like style transfer would fall flat on a binary image.
Source images in a posterized style, with large flat surfaces in unified colors, tend to yield great results with few artifacts. With that in mind, I find the raw "21" logo to be a rather good source image. Two colors is minimalist, but it's a choice. I'm sure GP went for simplicity quite deliberately.