Check out example codes for "change border color of TextField in flutter". It will help you in understanding the concepts better.

Code Example 1

decoration: new InputDecoration(
                border: new OutlineInputBorder(
                    borderSide: new BorderSide(color: Colors.teal)),
                hintText: 'Tell us about yourself',
                helperText: 'Keep it short, this is just a demo.',
                labelText: 'Life story',
                prefixIcon: const Icon(
                  Icons.person,
                  color: Colors.green,
                ),
                prefixText: ' ',
                suffixText: 'USD',
                suffixStyle: const TextStyle(color: Colors.green)),
          ),

Code Example 2

TextFormField(
        decoration: InputDecoration(
          labelText: "Resevior Name",
          fillColor: Colors.white,
          focusedBorder:OutlineInputBorder(
            borderSide: const BorderSide(color: Colors.white, width: 2.0),
            borderRadius: BorderRadius.circular(25.0),
          ),
        ),
      )

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