When I was using the new Thunkable (the beta one). I was not able to change the background colour.
Hey @code.thunk16n,
Welcome to the Community! 
This property isn’t available in the new UI right now. I’ve adde your request to the FAQs here: 👩🎨 The future of UI design in Thunkable - #102 by domhnallohanlon
We’re prioritising bug fixes and functionality improvements so hopefully this can get added in soon.
What sort of app are you working on and is the absence of a Text Input background colour property blocking you from making progress on the rest of your app?
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Also currently building an app w/ the new drag and drop builder and definitely hindered by the missing ability to adjust background colours and text sizes of the input fields. Would highly appreciate this being fixed! 
Best
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Do you have any ideas of the roadmap ?