For themes that are more conceptual (emotions, friendship, etc.) than practical (flowers, colors, etc.), look to Subject Tags in the library catalog to find books. Use the Advanced Search, select Subject, then search using your theme.
When you find something good, scroll down to the Subject Tags, and click on the right one. You will get a list with everything tagged with that Subject Tag.
When looking for picturebooks, don't discount nonfiction. There can be highly illustrated, age appropriate picturebook-like books in nonfiction as well.
Get creative! Think about themes that are related to your theme. If my theme is Flowers, I might also find things in Gardening, Nature, Spring, or Bees.