We give a color clue to the users so they can better tell if they're
sending an internal note or a public message.
Dislaimer: we use the `has()` pseudo class mainly because is simpler to
implement and the support is quite extended right now. Even Firefox will
catch up soon (now it has support behing a config flag).
So users with compatible browser will enjoy it, while others will just stay
as they were.
TT44953
