saveTheme abstract method
- required ZetaThemeData themeData,
 - required ThemeMode themeMode,
 - required ZetaContrast contrast,
 
Saves the provided theme data as the application's theme.
saveTheme is a method used to save the current theme data.
Takes a ZetaThemeData object that represents the theme to be saved.
Returns a Future that completes when the theme data has been successfully saved.
Implementation
Future<void> saveTheme({
  required ZetaThemeData themeData,
  required ThemeMode themeMode,
  required ZetaContrast contrast,
});