PIVOT-689, darken color used in tooltip background, to make text readable even in this color scheme.
This is a backport from trunk, but only for this change (this issue still doesn't fit in 2.0.x because of some incompatible changes).

git-svn-id: https://svn.apache.org/repos/asf/pivot/branches/2.0.x@1539543 13f79535-47bb-0310-9956-ffa450edef68