How is the motif of darkness woven into A Tale of Two Cities?

The darkness motif with its images of shadows in
A Tale of Two Cities points to the inscrutable quality of human nature along
with a sense of foreboding and mystery and often gloom.  In Chapter 3 of Book the First,
for instance, Dickens digresses from his narrative to reflect upon the fact that "every
human creature is constituted to be that profound secret and mystery to every other." 
Certainly, many of Dickens's characters are, indeed, do have a darker side to
them:

• Jerry Cruncher is a "resurrection man" at
  night


• Mr. Lorry is lost in the dark confines of Tellson's
  bank where he is figuratively buried from others.  Once he becomes involved with the
  Manette family he is happier for a time; however, the worries of Mr. Lorry for his
  friends in France increase, especially when Dr. Manette's letter implicates Charles
  Darnay.  In Chapter 18 of Book the Second, Mr. Lorry's concern for Manette increases,
  his "hope darknened and his heart grew
  heavier."

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With a hope ever darkening, and with a heart
always growing heavier and heavier, Mr. Lorry passed through this anxious
time.

• Dr. Manette
  has a dark part of his life that "casts a shadow beyond" himself that falls upon his
  son-in-law.  In Chapter 10 of Book the Third, "The Substance of a Shadow" is revealed
  and implicates Charles Darnay as the son of one of the dastardly Evremonde twins who
  have Manette imprisoned.  After his letter is read to the revolutionaries, "a shadow"
  falls over Manette and he seems lost again as in his dark days of
  imprisonment.


• Charles Darnay has a dark past inherited
  from his father that he seeks to escape by coming to England after changing his
  name.


• Madame Defarge certainly has a dark secret in her
  heart with which she harbors her plan of vengeance.  In Chapter 5 of Book the Third, as
  Lucie stands in the fresh white snow, Madame Defarge passes by "like a shadow over the
  white road" and she includes Lucie and her child in the deadly plan.  In Chapter 3
  entitled "The Shadow," in Book the Third, Madame Defarge knits Lucie and her child into
  the register of death:

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The shadow attendant on Madame Defarge and her
party seemed to fall so threatening and dark on the
child.... 

• Later
  in the Chapter entitled "Darkness," Chapter 12 of Book the Third, Madame Defarge's dark
  plan to have Darnay executed is revealed. 


• Solomon
  Pross, beloved brother of Miss Pross, lives a dark life of espionage and
  treachery.


• Sydney Carton, ironically, presents his darker
  side to the world, and it is his more noble side that finally emerges in his
  self-sacrifice for the love of Lucie
  Manette.

Shadows continually fall throughout
A Tale of Two Cities, creating a sense of mystery, foreboding,
obscurity, and gloom.