Let's take a quick moment to define the literary device in question.
What is an idiom? An idiom is a phrase or saying whose meaning is unrelated to the individual words within itself but instead is hinged upon some sort of cultural meaning.
What idioms are in Harrison Bergeron? To be honest, there are very few idioms within Kurt Vonnegut's short story. It is mostly composed of metaphors and deep visual imagery. However, here are two idioms:
"Equal before God and the law" - This saying relates to sentiments within the Old Testament and the Declaration of Independence (USA). It's meaning is not necessarily related to the words directly within it but is a statement of overarching clarity and severity. There is no exception to such a rule.
"right back to the dark ages again" - Unless the reader has cultural knowledge of what the "dark ages" are, this saying is nonsensical. This era, often referred to as the Medieval time period, developed this nickname as a means to contrast it to the period of enlightenment, or the Renaissance. The dark ages were not literally dark but were meant to be ignorant and violent.