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CustomTkinter includes a powerful theming system that controls the colors and visual styling of all widgets. Themes define separate color schemes for light and dark appearance modes.

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Built-in Themes

CustomTkinter comes with four built-in color themes:
  • blue (default): Professional blue accent colors
  • green: Fresh green accent colors
  • dark-blue: Deep blue accent colors
  • sweetkind: Custom theme with unique color palette

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Setting a Theme

Use set_default_color_theme() to change the active theme: 
import customtkinter as ctk

# Use the green theme
ctk.set_default_color_theme("green")

# Use the dark-blue theme
ctk.set_default_color_theme("dark-blue")
You must call set_default_color_theme() before creating any widgets. Theme changes do not affect existing widgets.

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Complete Example

import customtkinter as ctk

# Set theme BEFORE creating the app
ctk.set_default_color_theme("green")
ctk.set_appearance_mode("dark")

class App(ctk.CTk):
    def __init__(self):
        super().__init__()
        
        self.title("Theme Demo")
        self.geometry("500x400")
        
        # Frame
        self.frame = ctk.CTkFrame(self)
        self.frame.pack(pady=20, padx=20, fill="both", expand=True)
        
        # Label
        self.label = ctk.CTkLabel(
            self.frame,
            text="Green Theme Example",
            font=("Roboto", 24)
        )
        self.label.pack(pady=20)
        
        # Buttons showcase theme colors
        self.button = ctk.CTkButton(
            self.frame,
            text="Primary Button"
        )
        self.button.pack(pady=10)
        
        # Progress bar
        self.progress = ctk.CTkProgressBar(self.frame)
        self.progress.pack(pady=10)
        self.progress.set(0.7)
        
        # Slider
        self.slider = ctk.CTkSlider(self.frame)
        self.slider.pack(pady=10)
        self.slider.set(0.5)

if __name__ == "__main__":
    app = App()
    app.mainloop()

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Theme File Structure

Themes are stored as JSON files in the assets/themes/ directory. Here’s what a theme file looks like: 
{
  "CTkButton": {
    "corner_radius": 6,
    "border_width": 0,
    "fg_color": ["#2CC985", "#2FA572"],
    "hover_color": ["#0C955A", "#106A43"],
    "text_color": ["gray98", "#DCE4EE"]
  },
  "CTkFrame": {
    "corner_radius": 6,
    "border_width": 0,
    "fg_color": ["gray86", "gray17"]
  }
}
Color arrays like ["#2CC985", "#2FA572"] contain two values: [light_mode_color, dark_mode_color].

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Creating Custom Themes

You can create your own theme by providing a path to a custom JSON file: 
import customtkinter as ctk

# Load custom theme from file
ctk.set_default_color_theme("/path/to/my_theme.json")

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Custom Theme Example

Create a file called my_theme.json: 
{
  "CTk": {
    "fg_color": ["#F5F5F5", "#1A1A1A"]
  },
  "CTkButton": {
    "corner_radius": 8,
    "border_width": 0,
    "fg_color": ["#FF6B6B", "#EE5A52"],
    "hover_color": ["#FF5252", "#D84A47"],
    "border_color": ["#3E454A", "#949A9F"],
    "text_color": ["#FFFFFF", "#FFFFFF"],
    "text_color_disabled": ["gray74", "gray60"]
  },
  "CTkLabel": {
    "corner_radius": 0,
    "fg_color": "transparent",
    "text_color": ["#2D2D2D", "#E0E0E0"]
  },
  "CTkEntry": {
    "corner_radius": 8,
    "border_width": 2,
    "fg_color": ["#FFFFFF", "#2D2D2D"],
    "border_color": ["#CCCCCC", "#4A4A4A"],
    "text_color": ["#2D2D2D", "#E0E0E0"],
    "placeholder_text_color": ["gray52", "gray62"]
  }
}
Then load it in your application: 
import customtkinter as ctk
import os

# Get the directory of the current script
script_dir = os.path.dirname(os.path.abspath(__file__))
theme_path = os.path.join(script_dir, "my_theme.json")

ctk.set_default_color_theme(theme_path)

app = ctk.CTk()
app.mainloop()

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Themed Widget Properties

Each widget type can have the following theme properties:
  • fg_color: Foreground/background color (array for light/dark)
  • text_color: Text color
  • border_color: Border color
  • corner_radius: Corner radius in pixels
  • border_width: Border width in pixels
  • hover_color: Color when hovering (for interactive widgets)
CTkButton, CTkOptionMenu
  • text_color_disabled: Text color when disabled
CTkEntry, CTkTextbox, CTkComboBox
  • placeholder_text_color: Placeholder text color
CTkSwitch, CTkSlider, CTkProgressBar
  • progress_color: Color of the progress/active portion
  • button_color: Color of the draggable button/knob
  • button_hover_color: Button color on hover
CTkCheckBox, CTkRadioButton
  • checkmark_color: Color of the checkmark/indicator

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Platform-Specific Themes

Themes can specify different values for different operating systems: 
{
  "CTkFont": {
    "macOS": {
      "family": "SF Display",
      "size": 13,
      "weight": "normal"
    },
    "Windows": {
      "family": "Roboto",
      "size": 13,
      "weight": "normal"
    },
    "Linux": {
      "family": "Roboto",
      "size": 13,
      "weight": "normal"
    }
  }
}
CustomTkinter automatically selects the appropriate platform-specific values when loading the theme.

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Theme Manager Internals

The ThemeManager class handles theme loading:
  1. Built-in Detection: Checks if the theme name matches a built-in theme (blue, green, dark-blue, sweetkind)
  2. File Loading: Loads the JSON file from the built-in directory or custom path
  3. Platform Filtering: Extracts platform-specific values for the current OS
  4. Name Normalization: Fixes inconsistencies (e.g., CTkCheckbox → CTkCheckBox)
  5. Storage: Stores the theme data in ThemeManager.theme dictionary
All widgets read their default values from this dictionary when they are created.
Professional blue accent colors (#3B8ED0 in light, #1F6AA5 in dark). Great for business and productivity applications.

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Best Practices

  1. Set theme early: Always call set_default_color_theme() before creating any windows or widgets
  2. Test both modes: Verify your theme looks good in both light and dark appearance modes
  3. Use semantic colors: Name your colors based on purpose (primary, secondary, accent) rather than specific hues
  4. Provide defaults: Include all widget types in your custom theme to ensure consistency
  5. Consider accessibility: Ensure sufficient contrast between text and background colors
  • set_default_color_theme(color_string) - Set the color theme
  • set_appearance_mode() - Set light/dark mode (see Appearance Modes)

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