Динамическое изменение шрифта

from tkinter import *
from tkinter.ttk import *
 
from time import strftime
 
root = Tk()
#popup_window = Tk()
root.title("pyClock")
root.resizable(width=True, height=True)
 
size = 50
fnt = "Arial"
 
########################################
 
mainmenu = Menu(root)
root.config(menu=mainmenu)
 
def exit():
    root.quit()
 
def about():
    pass
 
def help():
    pass
 
def time():
    string = strftime('%H: %M: %S')
    label.config(text=string)
    label.after(900, time)
 
#TODO Добавить изменение формата 24/12 часов
formatmenu = Menu(mainmenu, tearoff=0)
formatmenu.add_command(label="24")
formatmenu.add_command(label="12")
 
programmenu = Menu(mainmenu, tearoff=0)
programmenu.add_command(label="About", command=about)
programmenu.add_command(label="Help", command=help)
programmenu.add_separator()
programmenu.add_command(label="Exit", command=exit)
 
mainmenu.add_cascade(label="Program",
                     menu=programmenu)
mainmenu.add_cascade(label="Format",
                     menu=formatmenu)
 
########################################
 
label = Label(root, font=(fnt, size), foreground="white")
label.pack()
time()
root.mainloop()