CODE EXAMPLE FOR DART

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),
  ),
)