Cannot override Themes\Generic.xaml

[nadilas]

Hello,

maybe I have spent too much time on this and am too close to see. How can I override the following colors?

    <SolidColorBrush
        x:Key="selectionBorderColor"
        Color="Blue"
        />
    
    <SolidColorBrush
        x:Key="selectionBorderBackgroundColor"
        Color="Blue"
        Opacity="0.25"
        />

window.TryFindResource() returns with null for these keys.

Thanks a lot