Return a dynamic resource from a converter

I want to change the color of a WPF control depending on the state of a bool, in this case the state of a checkbox. This works fine as long as I'm working with StaticResources:

My control

<TextBox Name="WarnStatusBox" TextWrapping="Wrap" Style="{DynamicResource StatusTextBox}" Width="72" Height="50" Background="{Binding ElementName=WarnStatusSource, Path=IsChecked, Converter={StaticResource BoolToWarningConverter}, ConverterParameter={RelativeSource self}}">Status</TextBox>

My converter:

[ValueConversion(typeof(bool), typeof(Brush))]

public class BoolToWarningConverter : IValueConverter
{
    public FrameworkElement FrameElem = new FrameworkElement();

    public object Convert(object value, Type targetType,
        object parameter, CultureInfo culture)
    {                      
        bool state = (bool)value;
        try
        {              
            if (state == true)
                return (FrameElem.TryFindResource("WarningColor") as Brush);
            else
                return (Brushes.Transparent);
        }

        catch (ResourceReferenceKeyNotFoundException)
        {
            return new SolidColorBrush(Colors.LightGray);
        }
    }

    public object ConvertBack(object value, Type targetType,
        object parameter, CultureInfo culture)
    {
        return null;
    }
}

The problem is that I have several definitions of the Resource "WarningColor" dependant on setting day mode or night mode. These events does not trig the WarningColor to change. Is there a way to make the return value dynamic or do I need to rethink my design?