Switch a message view to light elastic

[Windkracht8]

For some html emails the dark skin makes it difficult to read the email. A toggle to switch the message view to the light (default) elastic skin would be nice to make it easier to read.
I have a working prototype for this, if you are interested in this I will clean it up a bit and share the code.

[tborychowski]

Yes, please. That would be great.

[Windkracht8]

layout.html.diff.txt
message.html.txt
darklighttoggle.js.txt

The text Light and Dark are now hardcoded, to translate that into all available languages was a bit too much for me.

[tborychowski]

That's added now, thanks for your contribution!