This is a paid asset, but now you can download 100 Polygon Backgrounds for FREE, but please remember this package is provided only for learning purposes or to be able to test before buying the product, NOT FOR COMMERCIAL PURPOSES.
Detail this asset from Unity Store: Original Link
100 Polygon Backgrounds v1.0
Discover the beautiful pack of High-Resolution Backgrounds!
They are versatile, and you can use them in many designs. Make awesome designs that will not only catch the eye of everyone but will allow you to work with your imagination to build something playful and excellent. These backgrounds are the kind of resource that never goes old, and you can use them repeatedly.
Use them for brand work, print design, and many more to play with and save a lot of time.
What you will get:
100 Unique Backgrounds
Extended license – create and use as many of these fonts as you’d like on your own personal projects as well as any commercial ones for clients.