import tkinter as tk
from tkinter import ttk

root = tk.Tk()

# Pack a big frame so, it behaves like the window background
big_frame = ttk.Frame(root)
big_frame.pack(fill="both", expand=True)

# Set the initial theme
root.tk.call("source", "sun-valley.tcl")
root.tk.call("set_theme", "light")

def change_theme():
    # NOTE: The theme's real name is sun-valley-<mode>
    if root.tk.call("ttk::style", "theme", "use") == "sun-valley-dark":
        # Set light theme
        root.tk.call("set_theme", "light")
    else:
        # Set dark theme
        root.tk.call("set_theme", "dark")

# Remember, you have to use ttk widgets
button = ttk.Button(big_frame, text="Change theme!", command=change_theme)
button.pack()

root.mainloop()