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The use of the same black outline on all non-highlighted edges creates an unwanted optical broken-line effect (at least for me). I suggest that you instead draw the parts of the outline that is not adjacent to the bright background (i.e. not an enscapsulating out-line) with a color in-between the outline and the shadowside in brigthness (like grey [48,48,48]).
I love MC Escher. Good work ;)