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
Merge "Backport information boxes' styles from vforms to shared CSS"