Ryan,
Thanks for your feedback, I find out it have some bug to set the foreground when you use the theme.
So if you want to change the Foreground of the Scale Line, we'd suggest you to do the following:
1. Use explicit style in your theme, which means that give the style that targets TextBlock an explicit Key and refer to that key whenever needed. This way, we can prevent the ScaleLine from being affected by the theme.
2. Set the Foreground property to whatever value you want. For example: this.Map.MapTools.ScaleLine.Foreground = new SolidColorBrush(Colors.Black);
Any more questions please feel free to let me know.
Regards,
Gary