Flutterの3.3以降、ElevatedButtonのカスタムオプションで属性が変更されました。

ElevatedButton(
  style: ElevatedButton.styleFrom(
    // 以前は primary属性
    backgroundColor: Colors.red,
    // 以前は　　onPrimary属性
    foregroundColor: Colors.white,
  ),
  onPressed: () { },
  child: Text('ElevatedButton with custom foreground/background'),
)

FlatButton RaisedButton OutlineButtonウィジェットは廃止され、新しくTextButton ElevatedButton OutlinedButton になりましたので、使えそうなボタンをコピペできるようにまとめました。

ElevatedButton(
                  child: const Text('Button'),
                  style: ElevatedButton.styleFrom(
                    backgroundColor: Colors.blueAccent,
                    foregroundColor: Colors.white,
                  ),
                  onPressed: () {},
                )

ElevatedButton(
                  child: const Text('Button'),
                  style: ElevatedButton.styleFrom(
                    backgroundColor: Colors.blueAccent,
                    foregroundColor: Colors.white,
                    shape: const StadiumBorder(),
                  ),
                  onPressed: () {},
                )

OutlinedButton(
                  child: const Text('Button'),
                  style: OutlinedButton.styleFrom(
                    foregroundColor: Colors.black,
                    shape: RoundedRectangleBorder(
                      borderRadius: BorderRadius.circular(10),
                    ),
                    side: const BorderSide(color: Colors.blueAccent),
                  ),
                  onPressed: () {},
                )

OutlinedButton(
                  child: const Text('Button'),
                  style: OutlinedButton.styleFrom(
                    foregroundColor: Colors.black,
                    shape: const StadiumBorder(),
                    side: const BorderSide(color: Colors.blueAccent),
                  ),
                  onPressed: () {},
                )

ElevatedButton(
                  child: const Text('Button'),
                  style: ElevatedButton.styleFrom(
                    backgroundColor: Colors.grey[200],
                    foregroundColor: Colors.blueAccent,
                  ),
                  onPressed: () {},
                )

ElevatedButton.icon(
                  icon: const Icon(
                    Icons.star_outline,
                    color: Colors.blueAccent,
                  ),
                  label: const Text('Button'),
                  style: ElevatedButton.styleFrom(
                    backgroundColor: Colors.grey[200],
                    foregroundColor: Colors.blueAccent,
                  ),
                  onPressed: () {},
                )

ElevatedButton(
                  child: const Text('Button'),
                  style: ElevatedButton.styleFrom(
                    backgroundColor: Colors.blue,
                    foregroundColor: Colors.white,
                    shape: BeveledRectangleBorder(
                      borderRadius: BorderRadius.circular(10),
                    ),
                  ),
                  onPressed: () {},
                )

ElevatedButton(
                  child: const Text('Button'),
                  style: ElevatedButton.styleFrom(
                    backgroundColor: Colors.blue,
                    foregroundColor: Colors.white,
                    shape: BeveledRectangleBorder(
                      borderRadius: BorderRadius.circular(10),
                    ),
                  ),
                  onPressed: () {},
                )

ElevatedButton(
                  style: ElevatedButton.styleFrom(
                    shape: RoundedRectangleBorder(
                      borderRadius:
                          BorderRadius.only(topRight: Radius.circular(24)),
                    ),
                  ),
                  onPressed: () {},
                  child: Text('Button'),
                )

ElevatedButton(
                  onPressed: () {},
                  child: Text("Button"),
                  style: ElevatedButton.styleFrom(
                      shape: RoundedRectangleBorder(
                    borderRadius: BorderRadius.circular(30),
                  )),
                )

ElevatedButton(
                  onPressed: () {},
                  child: Text("Button"),
                  style: ElevatedButton.styleFrom(
                      shape: RoundedRectangleBorder(
                          borderRadius: BorderRadius.only(
                              topLeft: Radius.circular(20),
                              topRight: Radius.elliptical(10, 40)
                              //  bottomLeft:, bottom left
                              // bottomRight: bottom right
                              ))),
                )

ElevatedButton(
                  child: const Text('Btn'),
                  style: ElevatedButton.styleFrom(
                    backgroundColor: Colors.blue,
                    foregroundColor: Colors.white,
                    shape: const CircleBorder(
                      side: BorderSide(
                        color: Colors.blue,
                        width: 1,
                        style: BorderStyle.solid,
                      ),
                    ),
                  ),
                  onPressed: () {},
                )

TextButton(
                  child: const Text('Button'),
                  style: TextButton.styleFrom(
                    foregroundColor: Colors.black,
                  ),
                  onPressed: () {},
                )

 TextButton(
                  style: ButtonStyle(
                    foregroundColor: MaterialStateProperty.all<Color>(
                        Colors.lightBlueAccent),
                    overlayColor: MaterialStateProperty.resolveWith<Color?>(
                      (Set<MaterialState> states) {
                        if (states.contains(MaterialState.hovered))
                          return Colors.blue.withOpacity(0.04);
                        if (states.contains(MaterialState.focused) ||
                            states.contains(MaterialState.pressed))
                          return Colors.blue.withOpacity(0.12);
                        return null; // Defer to the widget's default.
                      },
                    ),
                  ),
                  onPressed: () {},
                  child: Text('Button'),
                )

TextButton(
                  style: ButtonStyle(
                    foregroundColor: MaterialStateProperty.all<Color>(
                        Colors.lightBlueAccent),
                  ),
                  onPressed: () {},
                  child: Text(
                    'Button',
                    style: TextStyle(
                        fontSize: 20, decoration: TextDecoration.underline),
                  ),
                )

OutlinedButton(
                  style: OutlinedButton.styleFrom(
                    shape: StadiumBorder(),
                    side: BorderSide(width: 2, color: Colors.lightBlueAccent),
                  ),
                  onPressed: () {},
                  child: Text(
                    'Button',
                    style: TextStyle(
                        fontSize: 16, decoration: TextDecoration.underline),
                  ),
                )

ElevatedButton(
                  child: const Text('Button'),
                  style: ElevatedButton.styleFrom(
                    backgroundColor: Colors.lightBlueAccent,
                    foregroundColor: Colors.white,
                    elevation: 16,
                  ),
                  onPressed: () {},
                )

ElevatedButton(
                  child: Container(
                    decoration: BoxDecoration(
                      gradient: LinearGradient(
                        colors: <Color>[
                          Colors.lightBlueAccent,
                          Colors.blue,
                          Colors.blueAccent,
                        ],
                      ),
                    ),
                    padding: const EdgeInsets.all(10),
                    child: const Text('Button'),
                  ),
                  style: ElevatedButton.styleFrom(
                    padding: const EdgeInsets.all(0),
                  ),
                  onPressed: () {},
                )