Have any Christian theologians discussed whether Jesus resembled Joseph? Mainstream Christians believe that Jesus was the son of God the Father and Mary, a virgin. So my question is, have any Christian theologians discussed whether Jesus only resemble his mother Mary, or whether he also resembled his adopted father Joseph?

God could easily have arranged it so that Jesus resembled Joseph, even if Joseph played no role in the conception of Jesus.
Not that I am aware of.  Since Matthew 1:18 and Luke 1:35 make it clear that no human father was involved in the conception of Jesus, then what Joseph looked like would have no bearing on what Jesus looked like, apart from the fact that they were both Jewish. Jospeh was Jesus' adoptive father.

The very first verse of the New Testament clearly proclaims the Jewish ethnicity of Jesus. “The book of the genealogy of Jesus Christ, the son of David, the son of Abraham” (Matthew 1:1). 

It is evident from passages like Hebrews 7:14 that Jesus descended from the tribe of Judah, from which the name "Jew" comes: “For it is clear that our Lord descended from Judah.” 

The genealogy of Mary, Jesus' physical mother, shows that she was a direct descendant of King David.  This is in Luke chapter 3 which establishes without any doubt that Jesus was a Jew ethnically.

Since the Bible is silent about the physical appearance of Jesus, then there would be no point in anybody speculating if Jesus physically resembled his adoptive father. I have been unable to find any references to Christian discussion on this, only that Christian theologians agree Joseph was not Jesus' physical father. 

EDIT: With regard to the question of Jesus’ genetic makeup, it is important to understand that the Bible tells us the the pre-mortal Jesus was the eternal and uncreated Word of God (John 1:1-3, 14). It was the eternal Word of God who left heaven in order to become the embryo in Mary’s womb. This pre-existent one had to empty Himself of the glory He had in the Godhead, alongside the Father and the Holy Spirit, to add human nature to His divine nature.

Mary’s contribution was the humanity of Jesus. The Formula of Chalcedon (451) speaks of Jesus as ‘consubstantial with us according to the manhood;’ the Athanasian Creed specifically links his manhood to the substance of his mother and that later, Protestant creeds do the same. The Westminster Confession (VIII.II), speak of him as “conceived... in the womb of the Virgin Mary, of her substance.”

Jesus had the same anatomy, physiology, biochemistry, central nervous system and the same basic genetic code as all humans.  Through Mary’s umbilical cord, he was that particular baby, the son of that particular woman who was the bearer of the whole previous genetic history of her people and the recipient of innumerable hereditary features.  He was a unique genotype precisely because Mary contributed at least half his chromosomes (as any human mother would). How the rest were contributed remains a mystery.

The one certainty is that Mary could not herself have contributed the sex-determining chromosome, Y, which is always provided by the biological father. As it was a divine act of the Holy Spirit, perhaps those chromosomes normally derived from the male parent were provided that way, the divine act which fertilized the ovum simultaneously creating twenty-three chromosomes complementary to those derived from the mother.  What we do know is that Jesus was not a clone of his mother, because then he would not have been male but would have been female. The incarnation was a unique event and humans cannot possibly give a biological break-down of every aspect of this unseen miracle in Mary’s womb. The Bible does not go into any details. 

Sources: The Person of Christ by Donald Macleod p 162 (Christian Focus 1998). Sexuality &amp; God in EL Mascal, Whatever Happened to the Human Mind? pp 128-155 (SPCK, London 1980)