Fixes #1067

I figured out that Blue was the only accent with a darker HighlightColor than the AccentColors.  It is about 26% darker.  I modified all other accents to have their HighlightColor be 26% darker than their AccentColor.  This seems to fix the issue with GroupBoxes on an accented flyout.

I added a GroupBox to the accent flyout on the flyout demo to illustrate the fixed accents.

I also added a new accent named Taupe.