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style = ttk.Style(root)
style.theme_use("clam")
style.configure("Treeview.Heading", background="black", foreground="white")
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root.configure(background="black")
# Or
root["bg"] = "black"
# Or more generally
<widget>.configure(background="black")
# Colors that work are those defined by the system (e.g. lots of named ones)
# or hex values, which can be of the forms #RGB, #RRGGBB, and #RRRRGGGGBBBB.
# Colors List: https://www.plus2net.com/python/tkinter-colors.php