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CustomTkinter provides functions to control the appearance mode (light/dark theme) of your application. The appearance mode affects the color scheme of all CustomTkinter widgets.

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set_appearance_mode()

Set the appearance mode for the entire application. 
import customtkinter as ctk

# Set to dark mode
ctk.set_appearance_mode("dark")

# Set to light mode
ctk.set_appearance_mode("light")

# Follow system theme
ctk.set_appearance_mode("system")

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Parameters

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mode_string
str
required
The appearance mode to set. Available options:
  • "light" - Force light mode for all widgets
  • "dark" - Force dark mode for all widgets
  • "system" - Automatically follow the system’s appearance mode (default)
The value is case-insensitive.

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Behavior

  • When set to "system", CustomTkinter automatically detects and follows your operating system’s appearance mode
  • Changes apply immediately to all existing CustomTkinter widgets
  • The setting persists for the lifetime of the application
  • System mode is monitored continuously and updates automatically when the OS theme changes

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get_appearance_mode()

Get the current appearance mode state. 
import customtkinter as ctk

mode = ctk.get_appearance_mode()
print(mode)  # Returns "Light" or "Dark"

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Returns

Type: str Returns the current active appearance mode:
  • "Light" - Application is currently in light mode
  • "Dark" - Application is currently in dark mode
Note: Even if the mode is set to "system", this function returns the actual resolved mode (“Light” or “Dark”), not “system”.

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Example

import customtkinter as ctk

root = ctk.CTk()

# Set dark mode
ctk.set_appearance_mode("dark")

# Create a toggle function
def toggle_mode():
    current = ctk.get_appearance_mode()
    if current == "Dark":
        ctk.set_appearance_mode("light")
    else:
        ctk.set_appearance_mode("dark")

# Button to toggle appearance mode
button = ctk.CTkButton(
    root,
    text="Toggle Theme",
    command=toggle_mode
)
button.pack(pady=20)

root.mainloop()

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Notes

  • System appearance mode detection requires the darkdetect package, which is installed automatically with CustomTkinter
  • On macOS and Windows, system mode detection works automatically
  • On Linux, system mode detection may have limited support depending on the desktop environment
  • The appearance mode is separate from the color theme (set via set_default_color_theme())
  • Appearance mode affects whether widgets use their light or dark color variants from the active theme

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