How do I change the overall theme of a tkinter application?

To change the theme, call .theme_use() with the theme's name as the argument.

From https://infohost.nmt.edu/tcc/help/pubs/tkinter/web/ttk-theme-layer.html

A number of operations related to themes require that you have available an instance of the ttk.Style() class (in the Python sense of class). For example, to obtain a list of the available themes in your installation:

>>> import ttk  # import tkinter.ttk as ttk for Python 3
>>> s=ttk.Style()
>>> s.theme_names()
('clam', 'alt', 'default', 'classic')

The .theme_names() method returns a tuple containing the names of the available styles. The 'classic' theme gives you the original, pre-ttk appearance.

To determine which theme you get by default, use the .theme_use() method with no arguments. To change the current theme, call this same method with the desired theme name as the argument:

>>> s.theme_use()
'default'
>>> s.theme_use('alt')
>>> s.theme_use()
'alt'

>>> from tkinter import ttk

>>> s=ttk.Style()

>>> s.theme_names() """======== if you are under win 8.1 you must see ..
 ('winnative', 'clam', 'alt', 'default', 'classic', 'vista', 'xpnative') you can use for example 'clam' ===== """

>>> s.theme_use('clam')

This post is pretty outdated, here is how you can easily set the theme in Python3 with just one line of code:

Add this below your "Tk()" line. For example:

window = Tk() # <--- Main window line

ttk.Style().theme_use('default') # <--- Change default to whichever theme you want to use.

Where 'default' is the name of the default theme. Change 'default' to any of the available themes that you like.

Here is a good list of themes with screenshots:

<-- Current themes as of 2020 -->

https://ttkthemes.readthedocs.io/en/latest/themes.html

Some themes from the list above are not included in the main tkinter download.

If this is the case, you can easily install the theme files that aren't included in the default ttk install by running this command:

python3 -m pip install git+https://github.com/RedFantom/ttkthemes

Hope this helped you!