Add class to Body of Canvas when dark mode is enabled

[HexaCubist]

I develop Design Blocks for Canvas, and our extension has a few issues with Better Canvas when dark mode is enabled. Ideally, we can add conditional CSS to our stylesheets so that when students have this tool installed, we can support dark mode on the inserted blocks. I have submitted a PR #60 that just adds and removes the class "bettercanvas--darkmode--enabled" when dark mode is turned on.