With all the underlying themes regarding freedom, civil rights, etc, what is the main unifying theme found in "Harrison Bergeron"?
The underlying and unifying theme in "Harrison Bergeron" is freedom, specifically the risks and threats to individual freedom.
George Bergeron isn't free - the "little mental handicap radio" and the "forty-seven pounds of birdshot in a canvas bag...padlocked around George's neck" insured that he wasn't free to think or do anything that was above average.
Hazel Bergeron isn't free. She has no external handicapping devices, but she also has no imagination to find anything to do but watch television without any real understanding or comprehension of what she is seeing. The performers she sees on the television are not free - they all bear evidence of their potential abilities in the weights and masks and other devices they are required to wear by the law of the land.
Harrison isn't free until he rebels. Until he declares himself "Emperor," he is the most extensively restricted and penalized of all. When he esapes from his handicapping devices, the reader understands the extent to which the government has gone to deprive him of his abilities and his freedom.
Harrison tore the straps of his handicap harness...straps guaranteed to support five thousand pounds...Harrison thrust his thumbs under the bar of the padlock that secured his head harness...Harrison smashed his headphones and spectacles against the wall. He flung away his rubber-ball nose, revealed a man that would have awed Thor, the god of thunder.
And in the end, Harrison's freedom causes his death. Handicapper General Diana Moon Glampers, who herself can never be free of her duties enforcing the requirements of the land, shoots Harrison before his expression of freedom even allows him to descend from his elevated position near the ceiling.
The lack of civil rights, the domination of every aspect of life by the government, the suppression of individual liberties, the brainwashing by the media - it all relates back to the theme of freedom and the lack thereof.
Another theme of "Harrison Bergeron" that is extremely pertinent to contemporary American society is the acceptance and value given to mediocrity. That Vonnegut considers this his prevailing idea is evident from his opening sentence, "The year was 2081, and everybody was finally equal." The culture of 2081 values this mediocrity to the point that people are willing to accept oppressive measures so that no one will be better than anyone else.
When Hazel watches the news announcer who is unable to read the news bulletin because of his severe speech impediment, an impediment that nevertheless has not prevented him from attaining his job, she remarks,
"That's all right--...he tried. That's the big thing. He tried to do the best he could with what God gave him."
Indeed, these words are a futuristic echoing of what is heard in present-day American society. Usually, such efforts are accompanied by a trophy or a certificate of participation. On maudlin television programs, audiences applaud the most pedestrian efforts of individuals participating in American Idol, The Voice, etc. because they came before the public and "tried."
On the other hand, competition is discouraged, and the modern emphasis is on the interpersonal interaction of players in a game, not on the winning. Society is lowered to meet the Handicapper General's standards in Harrison's world; in modern American society, no child is "left behind." The populace is desensitized, numbed by television and sound-bytes that shift and control thought rather than provoke it, a progressive desensitization promoted by technology.