Tkinter: TreeViw header background color change not working

Question

In my following code, background color of header is not changing. Although changed font color and size is working properly.

'''

import tkinter as tk
from tkinter import ttk

root = tk.Tk()
root.title('Treeview demo')
root.geometry('620x200')

columns = ('first_name', 'last_name', 'email')

style = ttk.Style()

style.configure("Treeview.Heading",font=('TkDefaultFont', 9,'bold'), background="grey",foreground="brown")

tree = ttk.Treeview(root, columns=columns, show='headings')

tree.heading('first_name', text='First Name')
tree.heading('last_name', text='Last Name')
tree.heading('email', text='Email')

contacts = []
for n in range(1, 100):
    contacts.append((f'first {n}', f'last {n}', f'email{n}@example.com'))

for contact in contacts:
    tree.insert('', tk.END, values=contact)

tree.grid(row=0, column=0, sticky='nsew')

root.mainloop()

'''