flutter textfield focus border color just bottom code example
Example: flutter text form field change underline color
decoration: InputDecoration(
enabledBorder: UnderlineInputBorder(
borderSide: BorderSide(color: theColor),
),
focusedBorder: UnderlineInputBorder(
borderSide: BorderSide(color: theColor),
),
border: UnderlineInputBorder(
borderSide: BorderSide(color: theColor),
),
)