$ python
Python 3.7.0 (default, Sep 15 2018, 19:13:07)
[GCC 8.2.1 20180831] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> from tkinter import Tk, Button, messagebox
>>> def Help():
... messagebox.showinfo('Window title', 'Sample text')
...
>>> root = Tk()
>>> root.option_add('*Dialog.msg.font', 'Helvetica 72')
>>> button = Button(root, text='Test button', font=('Helvetica', 36), command=Help)
>>> button.grid(row=2, column=0)
>>> root.mainloop()