Fixes #1067

I figured out that Blue was the only accent with a darker HighlightColor than the AccentColors.  Since all the other accent files had their AccentColor as an alpha of the Highlight color you could not see the AccentColor on top of the HighlightColor.  The HighlighColor needed to be a darker color than the accent colors.  It is about 26% darker in the blue accent.  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 modified the purple accent to be a little more different than violet.  Modified the yellow accent to help with contrast between the highlight and accent colors.

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.