Make messages color darker (#533).

Use medium-gray gray value for ul.messages rather than light-gray.
Difficult to be certain that this change won't make some other use
of ul.messages unreadable, but a brief search didn't turn up anything
obvious, and the change in value is smallish.
Merge branch '533-unreadable-message' of https://github.com/patternleaf/atlas into patternleaf-533-unreadable-message

Brings in more prominent message styles to address #533
Style form messages

Move the nice looking styles from the ``ul.messages`` element to the
``ul.messages li`` element. This allows multiple types of messages to
be shown with the appropriate styling, with each message nicely highlighted
in an element with a background color and rounded corners.

Addresses #533