Python Tkinter : How to change underground / background color of Treeview, Entry and other widgets

Question

By using wrong keyword I've stupidly struggled to change what we could call the first level of color of ttk widgets. It's not underground or underlayer but fieldbackground as most of you already know.
See below...

Answer 1

A little reminder :
background color is the color under the text in the widget
foreground color is the color of the text
fieldground color is the color of the place where the text will appear

This can be manage with Style:

self.MainTk = tkinter.Tk()
self.style = ttk.Style( self.MainTk )
self.style.configure("Treeview", fieldbackground = 'grey65')
self.style.configure("TEntry",  fieldbackground = 'grey65')

Note that some widgets need a T before the name in the configure phase.
see https://www.pythontutorial.net/tkinter/ttk-style/ for more infos.
In Treeview, it manage the Tree mode appearence when you have a single parent collapsed.

From digging old posts it seems that this may not work with some themes under some configuration. You'll find out.

WARNING : Since the error is often made even if it's logic, the widgets must be ttk not tkinter so you must use

self.MyEntry = ttk.Entry(MainTk)

instead of

self.MyEntry = tkinter.Entry(MainTk)

or

self.MyEntry = tk.Entry(MainTk)

if you have import tkinter as tk. Here lie the most common mistake I think.