Using a StaticResource SolidColorBrush to define the Gradient Stop Colors
Ok, I found the problem:
Using Color and not SolidColorBrush..
<Color x:Key="colorbrushMedium">#FF5A5A5A</Color><Color x:Key="colorbrushDark">#FF222222</Color><LinearGradientBrush> <GradientStop Color="{StaticResource colorbrushMedium}"/> <GradientStop Color="{StaticResource colorbrushDark}" Offset="1"/></LinearGradientBrush>This seems to solve my problem!
Use Binding to reference the color both in SolidColorBrush and in LinearGradientBrush:
<SolidColorBrush x:Key="stop1" Color="#FF5A5A5A"/><SolidColorBrush x:Key="stop2" Color="#FF222222"/><LinearGradientBrush x:Key="gradient"> <GradientStop Color="{Binding Source={StaticResource stop1},Path=Color}" Offset="0"/> <GradientStop Color="{Binding Source={StaticResource stop2},Path=Color}" Offset="1"/></LinearGradientBrush>