I'm sorry but in the One UI experience, the colour scheme is standard.
You can try Google Messenger or another safe SMS app for customization. Samsung messages for now, is stock colour only.
My friend has Verizon she has the update...and she can still customize her
background... Is this a carrier update issue?
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You can change it with themes or dark mode. No specific custom background anymore.
Textra is a good option, along with pulse and mood. However none will have RCS.