ThemeMode
        Inherits: Enum
Describes which theme will be used by Flet app.
Properties
- 
          DARK–Always use the dark mode (if available) regardless of system preference. 
- 
          LIGHT–Always use the light mode regardless of system preference. 
- 
          SYSTEM–Use either the light or dark theme based on what the user has selected in the 
Properties#
DARK = 'dark'
  
      class-attribute
      instance-attribute
  
#
    Always use the dark mode (if available) regardless of system preference.
LIGHT = 'light'
  
      class-attribute
      instance-attribute
  
#
    Always use the light mode regardless of system preference.
SYSTEM = 'system'
  
      class-attribute
      instance-attribute
  
#
    Use either the light or dark theme based on what the user has selected in the system settings.