There is no reference to a specific Keats poem in your question. So, I will try and give a general answer.
The Romantic themes in Keats are
1. The sensuality and picturesqueness, especially the synesthetic sensuousness of nature.
2. The flux of time and nature inscribed into it, producing a deep tragic melancholy.
3. The growth of the imaginative mind in the symptomatic inspirations of nature.
4. The tragic knowledge full of sorrow. Sorrow is wisdom.
5. The flight for the transcendental order only to return to the inevitable mortality of the world as in the Nightingale Ode.
6. The paradoxic poles of desire in between static eternity and dynamic mortality as in Bright Star and Grecian Urn Ode.
7. The intense focalization on subjectivity.
8. The antinomy between the ideal and the real, and experiments with the limits of imagination.