Three short lines give us a single picture: a sunflower seen up close. The "heart of darkness" is the dark center of the flower, where the seeds sit, and the poem describes it as "glowing orange," which is a surprising mix. Darkness and glow are normally opposites, so the center feels both heavy and warm at once. Then the petals appear as a "feathery halo," soft and light, circling that dark middle like a crown around something precious.
The speaker never says outright how they feel, but the word choices suggest quiet admiration. Calling the petals a halo makes the flower seem almost holy, and the phrase "heart of darkness" hints that beauty may hold something deeper or more mysterious inside it. The title, "On the Topic of Sunflowers," sounds casual, as if the speaker is simply musing. The mood is hushed and contemplative, with a small touch of awe at how much can be packed into such a brief look.