focusColor property
final
The fill color of the decoration's container when it has the input focus.
By default the focusColor is based on the current InputDecorationTheme.focusColor.
This focusColor is ignored by TextField and TextFormField because they don't respond to focus changes by changing their decorator's container color, they respond by changing their border to the focusedBorder, which you can change the color of.
The decoration's container is the area which is filled if filled is true and bordered per the border. It's the area adjacent to icon and above the widgets that contain helperText, errorText, and counterText.
Implementation
final Color? focusColor;