How to change the theme of my tkinter application in Python?

Below is an modified example using ttk widgets based on your code:

from tkinter import *import tkinter.ttk as ttkimport randomwindow = Tk()window.title("Running Python Script") # Create windowwindow.geometry('550x300') # geo of the windows=ttk.Style()s.configure("TButton", foreground="red", background="blue")list_themes = s.theme_names()current_theme = s.theme_use()#s.theme_use('classic')print(list_themes)def run():    if dd_owner.get() == "Spain":        print("spain")    # choose a theme randomly    theme = random.choice(list_themes)    print("theme:", theme)    s.theme_use(theme)# These are the option menusowner = ("Spain", "France", "Germany")dd_owner = StringVar(window)dd_owner.set(owner[0]) # the first value#w = OptionMenu(window, dd_owner, *owner)# use ttk.Combobox instead of OptionMenuw = ttk.Combobox(window, textvariable=dd_owner, values=owner)w.grid(row=0, column=1)#The run button#run_button = Button(window, text="Run application {}".format(dd_owner.get()), bg="blue", fg="white",command=run)# use ttk.Buttonrun_button = ttk.Button(window, text="Run application {}".format(dd_owner.get()), command=run)run_button.grid(column=0, row=2)# These are the titlesl1 = ttk.Label(window, text='Select Owner', width=15)l1.grid(row=0, column=0)mainloop()

When you click the button, it will change the theme randomly.