To design a GUI that offers both simplicity and extensive customization options:

1. Start with a simple, intuitive default layout.

2. Provide clear, concise options for basic customization (e.g., themes, font size).

3. Offer advanced customization options in a separate section (e.g., "Expert Mode").

4. Use clear, descriptive labels and tooltips for customization options.

5. Organize customization options into logical categories.

6. Implement a "reset to default" option.

7. Use visual cues (e.g., icons, color schemes) to differentiate between customization levels.

8. Make advanced options accessible but not overwhelming (e.g., collapsible sections).

9. Test with users to ensure simplicity and customization options meet their needs.

10. Balance simplicity and customization, avoiding overwhelming users with too many options.

By following these guidelines, you can create a GUI that caters to both novice users who value simplicity and power users who desire extensive customization options.