Final answer:
Shelley's central theme in his poetry, such as in "England in 1819", focuses on the decline of empires and the need for social and political reform.
Step-by-step explanation:
The central theme that Shelley conveys, particularly in the poem "England in 1819", is how all empires decline.
Shelley's harsh criticism of England in his poems is reflective of the grim realities of the era, which includes the Industrial Revolution, poverty among the working class, and a tumultuous political situation.
In "England in 1819", Shelley uses stark metaphors to describe the institutions of his country: the Church as a 'starved' entity, Parliament as 'time's worst statute', and the Army as blind 'leechlike' to their own freedom. Shelley's poem ends with the gleam of hope that arises from the 'new and nobler' times born from the ashes of the 'old, mad, blind, despised, and dying' age. This desperate cry for reform and recognition of inequality and corruption still resonates deeply in today's context.