Control on the right is `UIButton` with `.gray()` configuration.
Control on the left is my custom stepper with `UITextField` inside and two plain buttons around it.

I wanted to use exact same background on the stepper as on the button. It turns out that it's `UIColor.secondarySystemFill` which is (currently):

→ sRGB 0.470588 0.470588 0.501961 α0.16

or

→ RGB (120,120,128), 16%

Important bit is that this is with `baseBackgroundColor` set to `nil`.