I shelved this dark not because of Oxenbury's treatment, but rather because Carroll's writing is rather dark in tone. He does it well.
Oxenbury's illustrations are just what we would expect--very good figure drawings in her signature style. She chose not to depart from the conventional illustration tradition for this book, and every single picture maps almost 100% onto words from the text. I've always hoped someone would do something a little more complementary or counterpointy at least with the Father William verses, and these were the most representational of all. However, I expect editors don't want illustrators to take something that was once radical, now traditional, and complicate it again. I'd have liked to be a fly on the wall for that meeting.
At any rate, they gave her full rein on the graphic design. Wonderful paper, great wide margins and gutters, comfortably large print and many double page illustrations in full color. What a great project for Oxenbury!