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CustomTkinter provides a theme system that controls the color scheme of widgets. You can use built-in themes or load custom theme files.

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set_default_color_theme()

Set the default color theme for CustomTkinter widgets. 
import customtkinter as ctk

# Use built-in theme
ctk.set_default_color_theme("blue")  # Default theme

# Load custom theme
ctk.set_default_color_theme("/path/to/custom_theme.json")

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Parameters

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color_string
str
required
Either the name of a built-in theme or the path to a custom theme JSON file.Built-in themes:
  • "blue" - Default blue theme
  • "green" - Green color scheme
  • "dark-blue" - Dark blue variant
  • "sweetkind" - Custom sweetkind color scheme
Custom themes: Pass the absolute or relative path to a JSON theme file to load a custom color scheme.

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Behavior

  • This function should be called before creating any widgets
  • The theme affects widget colors in both light and dark appearance modes
  • Built-in themes are loaded from CustomTkinter’s assets directory
  • Custom theme files must follow CustomTkinter’s theme JSON structure

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Theme File Structure

Custom theme files are JSON files that define colors for different widget types and states. Here’s a basic structure: 
{
  "CTkButton": {
    "fg_color": ["#3B8ED0", "#1F6AA5"],
    "hover_color": ["#36719F", "#144870"],
    "border_color": ["#3E454A", "#949A9F"],
    "text_color": ["#DCE4EE", "#DCE4EE"]
  },
  "CTkLabel": {
    "fg_color": "transparent",
    "text_color": ["gray10", "gray90"]
  },
  "CTkFont": {
    "family": "Arial",
    "size": 13,
    "weight": "normal"
  }
}
Color values in arrays represent [light_mode_color, dark_mode_color].

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Platform-Specific Values

Themes can specify platform-specific values: 
{
  "CTkButton": {
    "corner_radius": {
      "macOS": 8,
      "Windows": 6,
      "Linux": 6
    }
  }
}

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Example

import customtkinter as ctk

# Set theme before creating widgets
ctk.set_default_color_theme("green")

root = ctk.CTk()

# These widgets will use the green theme
button = ctk.CTkButton(root, text="Green Button")
button.pack(pady=20)

label = ctk.CTkLabel(root, text="Green Theme Label")
label.pack(pady=20)

root.mainloop()

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Example: Custom Theme

import customtkinter as ctk
import json

# Create a custom theme file
custom_theme = {
    "CTkButton": {
        "fg_color": ["#FF6B6B", "#C92A2A"],
        "hover_color": ["#FF5252", "#A61E1E"],
        "text_color": ["white", "white"]
    },
    "CTkFrame": {
        "fg_color": ["#F8F9FA", "#212529"]
    }
}

# Save to file
with open("my_theme.json", "w") as f:
    json.dump(custom_theme, f, indent=2)

# Load the custom theme
ctk.set_default_color_theme("my_theme.json")

root = ctk.CTk()
root.mainloop()

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Notes

  • The theme defines the default colors; individual widgets can still override colors using their parameters
  • Built-in themes are located in customtkinter/assets/themes/
  • Theme changes only affect widgets created after set_default_color_theme() is called
  • The theme is separate from the appearance mode (light/dark)
  • For a complete theme structure, examine the built-in theme files in the CustomTkinter source code

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