Options for theme

[Sandip124]

this library can have property like theme and so that the look and feel can be changed in semantic way without even create custom CSS in the toastify CSS.

this.options.theme = options.theme|| Toastify.defaults.theme; // Default, Light, Dark

and also we can create a theme and can publish so that every body can use these theme directly using the toaster lib

also we can implement like success , danger info theme, and there the icon for it would be already set with proper data ,
this would have benefit when the end user consumes its API and they can directly call toastify .success or

Also for that we need some kind of invocation like toastify can directly invoke theme while initiating.