An interesting way of thinking about theme in Antigone is through an analysis of the contrast between Antigone and Ismene.

A defensible thesis statement could be:

Antigone is a proto-feminist heroine whose execution in a male-dominated culture was inevitable. 

Ismene is conventional, obedient to authority, and incapable of the independent thought and boldness that Antigone possesses, seen clearly in her line:

"We are only women, We cannot fight with men, Antigone!"

Antigone's response to her sister is

"You have made your choice, you can be what you want to be."

Even though she understands that defying Creon's edict and burying their brother, Polyneices, will result in her execution, Antigone chooses to do what her personal values and the gods deem correct:  bury her brother so that his soul will proceed to the underworld.  She rejects her sister (the sole remaining member of her nuclear family), her fiance, her future, and the rule of law in Thebes as declared by her uncle. 

The following thesis statement once upon a time served me well:

A major theme of Antigone is the conflict between religious law and man-made law.

Antigone believes in the supremacy of religious law and tradition.  Although King Creon has decreed that the traitor Polyneices should not be buried, Antigone insists on following the religious law requiring that females bury the deceased of their family.

The poet Teiraisias warns Creon:

You have dishonored a living soul with exile in the tomb,/hurling a member of this world below./You are detaining here, moreover/a dead body, unsanctified, and so unholy ,/a subject of the nethergods.

Creon, however, insists that Antigone has commited "open rebellion," and that if he does not put her to death he will betray his responsibility to his subjects:

Since I caught her, alone of all entire(665)
people, in open rebellion, I will not
make myself a liar to the city,
but kill her.