What? You're kidding me, why didn't I know about this!

I like it a lot. I just added a Stylish custom style so I have it looking exactly how I want.

I also like how it's an HTML page, so copying an image from anywhere and paste right in there works great. Does it properly as an HTML image tag with src to image source.

I'm guessing an additional greasemonkey script could save it to cloud with one click, or publish it somewhere.

I had the same reaction when I saw this! There are many great styles here:

https://coderwall.com/p/lhsrcq/one-line-browser-notepad

Ha! That's an admirably simple solution. Not quite a full replacement, but it gets you surprisingly far. :)