There are quite a few ways to add a background. Some less than common.
Fabric of your choice with velcro to attach.
Window film, privacy film - rice paper looks nice. You can up light the back with an LED
too or try poly film like for making/wrapping gift baskets.
Depending on the size of your tank, place mat or yoga mat, washable table runner, in any color or material you like, bamboo, grass cloth, burlap, shade fabric or outdoor patio fabric etc.
Steel /aluminum, other metals used for doors and decorative purposes as an example there a so many patterns.
M-D Building Products 12 in. x 24 in. Lincane Aluminum Sheet in Brass-56012 - The Home Depot
New to FW fish? First tank in a long time or EVER??? Need some info on keeping fish?
Start here - Aquarium Advice Article
FW Favorites right now Clown Loaches, any shrimp - SW Vase Red Mangroves