Some instance member can't be accessed using static access - flutter

Question

Earlier this week I followed some tutorial on Youtube for creating a flutter app, and I got some problems where it says that.
Instance member 'textTheme' can't be accessed using static access.
The problems occur at LandingPage.dart
#override
Widget build(BuildContext context) {
final ThemeData themeData = Theme.of(context);
return Container(
margin: const EdgeInsets.symmetric(vertical: 20),
child: Column(
children: [
Stack(
children: [
ClipRRect(
borderRadius: BorderRadius.circular(25.0),
child: Image.asset(itemData["image"])),
Positioned(
top: 15,
right: 15,
child: BorderBox(
width: 50,
height: 50,
child: Icon(Icons.favorite_border,color: COLOR_BLACK,
),
))
],
),
addVerticalSpace(15),
Row(
children: [
Text("${formatCurrency(itemData["amount"])}",
style: themeData.textTheme.headline1)
],
)
],
)
);}
textTheme.headline1 was imported from constants.dart.
import 'package:flutter/material.dart';
const COLOR_BLACK = Color.fromRGBO(48, 47, 48, 1.0);
const COLOR_GREY = Color.fromRGBO(141, 141, 141, 1.0);
const COLOR_WHITE = Colors.white;
const COLOR_DARK_BLUE = Color.fromRGBO(20, 25, 45, 1.0);
const TextTheme TEXT_THEME_DEFAULT = TextTheme(
headline1: TextStyle(
color: COLOR_BLACK, fontWeight: FontWeight.w700, fontSize: 26),
headline2: TextStyle(
color: COLOR_BLACK, fontWeight: FontWeight.w700, fontSize: 22),
headline3: TextStyle(
color: COLOR_BLACK, fontWeight: FontWeight.w700, fontSize: 20),
headline4: TextStyle(
color: COLOR_BLACK, fontWeight: FontWeight.w700, fontSize: 16),
headline5: TextStyle(
color: COLOR_BLACK, fontWeight: FontWeight.w700, fontSize: 14),
headline6: TextStyle(
color: COLOR_BLACK, fontWeight: FontWeight.w700, fontSize: 12),
bodyText1: TextStyle(
color: COLOR_BLACK, fontWeight: FontWeight.w500, fontSize: 14, height: 1.5),
bodyText2: TextStyle(
color: COLOR_GREY, fontWeight: FontWeight.w500, fontSize: 14, height: 1.5),
subtitle1: TextStyle(
color: COLOR_BLACK, fontSize: 12, fontWeight: FontWeight.w400),
subtitle2: TextStyle(
color: COLOR_GREY, fontSize: 12, fontWeight: FontWeight.w400)
);
const TextTheme TEXT_THEME_SMALL = TextTheme(
headline1: TextStyle(
color: COLOR_BLACK, fontWeight: FontWeight.w700, fontSize: 22),
headline2: TextStyle(
color: COLOR_BLACK, fontWeight: FontWeight.w700, fontSize: 20),
headline3: TextStyle(
color: COLOR_BLACK, fontWeight: FontWeight.w700, fontSize: 18),
headline4: TextStyle(
color: COLOR_BLACK, fontWeight: FontWeight.w700, fontSize: 14),
headline5: TextStyle(
color: COLOR_BLACK, fontWeight: FontWeight.w700, fontSize: 12),
headline6: TextStyle(
color: COLOR_BLACK, fontWeight: FontWeight.w700, fontSize: 10),
bodyText1: TextStyle(
color: COLOR_BLACK, fontWeight: FontWeight.w500, fontSize: 12, height: 1.5),
bodyText2: TextStyle(
color: COLOR_GREY, fontWeight: FontWeight.w500, fontSize: 12, height: 1.5),
subtitle1: TextStyle(
color: COLOR_BLACK, fontSize: 10, fontWeight: FontWeight.w400),
subtitle2: TextStyle(
color: COLOR_GREY, fontSize: 10, fontWeight: FontWeight.w400)
);
Thank You and have a good day:))