Feature Request: User stylesheet

The Calibre ebook viewer allows us to set a user stylesheet that gets applied to all books opened. I think the same concept in the editor would be useful, because it would allow us to more easily see the results of our edits.

Basically, if a user has a complex user stylesheet on the reader, then it's hard to tell if the edits are ending up looking right. After a while, you do get good at mentally changing the preview with what the user stylesheet does, but sometimes it's still tricky. The biggest one for me is that my user stylesheet has no space by default before/after <p> elements, so seeing if there is extra space when there should be (scene break, etc.) isn't easy.

I know I could simulate this by copying the user stylesheet and pasting it to the front of the stylesheet of the book I am editing, but then I need to remember to remove it before I save (since all my readers/devices have their own version of this stylesheet installed, tweaked for their idiosyncrasies).

Also, for some users, the "theme" functionality of the reader that allows quickly changing settings (which includes the user stylesheet) might be useful, but I suspect that's a lot more work.