What is the theme of the poem "The Bangle Sellers" by Sarojini Naidu?

I would say the theme of this poem is that there are common steps taken throughout the life of a woman. While I'm not suggesting that every woman must follow these steps, the poem represents three major moments in life that seem to define women. The bangles are used to represent those moments.

The first stanza is an introduction to the idea of the bangles representing the women's stages of life. They establish that the bangles are for "happy daughters and happy wives." This sets up for the rest of the poem, where the stages of lives are described by the colors of the bangles being sold by the bangle sellers to the various women.

The second stanza discusses women in adolescence, or, maidenhood. These women wear bangles of "silver and blue" and the entire section is tinged with nature references as the blooming plants are related to the blooming women.

The third stanza discusses women at the age of marriage.

Some, like the flame of her marriage fire,
Or, rich with the hue of her heart's desire,
Tinkling, luminous, tender, and clear,

These descriptions hone in on passion and tenderness, the qualities of a wife and someday a mother.

The last stanza discusses women who have had children and have families. These bangles are "purple and gold flecked grey." These are rich colors, representative of a woman who is rich in experience with a husband, family, and religion.

Hope this helps!