How does the Theban Plays relate to the concept that our fate is predetermined and cannot be changed no matter how hard somebody fights against...

In the end, Sopocles' trilogy demonstrates the
overwhelming power of fate.  The triloge reflects that the fate of the individual cannot
be changed and while individuals might try to avert it, the force of the divine is
overwhelmingly stronger than that of the individual's.  Oedipus learns this the most
painful way possible.  In his particular narrative, the majority of the play is spent
with him seeking to outwit his fate, to use his own sense of pride and accomplishment in
being able to cast aside his fate and will himself to success.  He fails in the most
reprehensible and pathetic of ways.  In Oedipus and Colonus, I think that it is Creon
who learns of the power of fate.  When Creon recognizes that Oedipus must be buried in
Thebes in order to bless it, he seeks to use his free will to take Oedipus by force.  He
also seizes his nieces, Oedipus daughters.  It is this hubris that again becomes
representative of how Sophocles believes that individuals are relatively weak in
comparison to their own fate.  This is proven in the case of Antigone, whose own
defiance to Creon's laws end up proving the fates correct in ruining his kingdom and his
own personal state of being.  The three works show that an individual has to accept and
make peace with their fate.  Nothing they can do with their free will can prove
otherwise, except for the demonstration of hubris that is
present.