The central theme of the poem "Amma" is aging and nostalgia.

1. Aging:
The poem explores the theme of aging through the perspective of the speaker's grandmother, referred to as "Amma." The poet highlights the physical and emotional changes that come with old age. For example, the poem mentions Amma's "silver hair" and her "wrinkled skin," which symbolize the passing of time and the effects of aging. These descriptions evoke a sense of fragility and vulnerability associated with old age.

2. Nostalgia:
The poem also delves into the theme of nostalgia, depicting the speaker's memories of their grandmother's house. The poet uses vivid and sensory imagery to create a nostalgic atmosphere. For instance, the poet mentions the "rustle of sarees," the "fragrance of jasmine flowers," and the "echo of laughter." These details evoke a sense of longing for the past and a yearning for the comfort and familiarity of the grandmother's house.

3. The bond between generations:
Another aspect of the central theme is the bond between generations. The poem portrays the close relationship between the speaker and their grandmother. The poet describes how Amma used to tell stories and sing lullabies, creating a sense of warmth and love. This bond is further emphasized through the use of the term "Amma," which is a term of endearment for a grandmother in many South Asian cultures.

4. Reflection on mortality:
Within the theme of aging, the poem also touches upon the transient nature of life and the inevitability of mortality. The poet mentions how Amma's voice "grows faint" and how her memories "fade away." These reflections on the passage of time evoke a sense of contemplation and emphasize the importance of cherishing and preserving memories before they are lost.

In conclusion, the central theme of the poem "Amma" is aging and nostalgia. Through vivid imagery and a focus on the bond between generations, the poem explores the physical and emotional changes that come with old age, evokes a sense of longing for the past, and reflects on the transient nature of life.