PIVOT-689, darken color used in tooltip background, to make text readable even in this color scheme.
Later I'll make other changes to this color scheme, for example to better test the themeIsDark flag (now false even here).

git-svn-id: https://svn.apache.org/repos/asf/pivot/trunk@1539410 13f79535-47bb-0310-9956-ffa450edef68