As a way into Rossetti's work, I would recommend a study of her first lines. It's quite revealing. Here is the Index of first lines from Elizabeth Jennings' collection: First Lines.
Give students one minute to scan through the list then ask them to spend one minute jotting down their first impressions. Go round the group and get one item or thought from each student. How far does this reveal Rossett's themes?
Alternatively, provide them with this collapsed version as a paper or electronic worksheet. There's also a worked example which might provide some starting points. Look at the repetitions of pronouns, for example. (It's all 'me me me'!)
Once you have done this, go to some of the poems and see where and why she uses dreams and roses, to what extent positive themes (like love) are qualified by negatives, how often love is related to love of God rather than humanity, or a specific person.
Here's a way of looking at the themes of her poetry.
A Royal Princess.
See also the page on Cousin Kate.