Public preview fade out effect

[bst1n]

I think this is an essential feature for any Ghost theme.

Without it, it's difficult for the reader to project the fact that the article has a sequel.

I've based myself on this forum thread and it's a code I've tested locally that works well.

https://forum.ghost.org/t/public-preview-fade-out-effect-removes-spaces-between-paragraphs/49867