Backport information boxes' styles from vforms to shared CSS

They're obviously prettier and non-intrusive enough to fit old
interfaces. The colors are changed to more pastel ones and the general
size of the boxes is reduced.

Also remove unnecessary bolds on the informations on
Special:Preferences and Special:ChangePassword.

Change-Id: Ieae62db1a124261ae7f5bf67aced8b84cfbadd3d
(cherry picked from commit 0b539d7fa1fc312b3be6804a2e51713da86fefee)