What is the "swelling act of the imperial theme"?
In Act 1, Scene 3 Macbeth says in an aside:
Two truths are told
As happy prologues to the swelling act
Of the imperial theme.
He is referring to the first two truths told to him by the three witches when he and Banquo encountered them on the "blasted heath." These weird sisters hailed him as Thane of Glamis and Thane of Cawdor. Macbeth was already Thane of Glamis, but evidently the witches would have had no way of knowing that. In fact, they should have had no way of recognizing him as Macbeth or Banquo as Banquo.
The really surprising truth was in hailing him as Thane of Cawdor, which was something he didn't know about yet. The fact that the weird sisters have prophetic powers makes a deep impression on Macbeth, who is already toying with the notion of finding a way to become king by somehow getting rid of Duncan and has been discussing his ambition with his wife.
The imperial theme means the attainment of the regal power and glory of the King of Scotland. He calls the other truths "prologues," as a metaphor suggesting that his two ascensions in noble status are acts in a drama which will be highlighted by the third and final act, or theme, which will be his coronation and installation as King of Scotland.
The most appropriate definition of the word "imperial" as used by Shakespeare is defined by The American Heritage Dictionary as:
Obsolete. Having supreme authority; sovereign.
"The swelling act of the imperial theme" refers to Macbeth's impending kingship, as predicted by the Weird Sisters. When Macbeth and Banquo first encounter the Sisters, the two warriors are skeptical of what seem to be wild predictions. Macbeth says, "By Sinel's death I know I am thane of Glamis / But how of Cawdor? the thane of Cawdor lives, / A prosperous gentleman; and to be king / Stands not within the prospect of belief, / No more than to be Cawdor."
However, as it quickly becomes evident that by some supernatural power the Three Witches do indeed know the future (it's almost immediately revealed that King Duncan has named Macbeth Thane of Cawdor because the previous holder of that office is a traitor), Macbeth begins to believe, and also begins to be seduced by the idea that he can become king.
In the time depicted in Shakespeare's play, royal succession was not automatic from father to son. Later in the play, Duncan does name his son Malcolm as his chosen successor, but among the bloody clansmen, that was no guarantee. Whoever could take and hold power would be king, and Macbeth has already proven himself a valiant soldier. The evil worm of ambition has begun to eat into his dreams, and the tragedy is set in motion.