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?