I am trying to make my application more customizable by allowing users to pick a color from a Color Picker dialog, and then changing the style of the application in real time (with DynamicResource)

How do I go about in changing specific resources that reside in the app.xaml ?

I have tried something like this but no luck (just a test):

var colorDialog = new CustomControls.ColorPickerDialog();
var dResult = colorDialog.ShowDialog();
var x = Application.Current.Resources.Values.OfType<LinearGradientBrush>().First();
x = new LinearGradientBrush();
x.GradientStops.Add(new GradientStop(colorDialog.SelectedColor,1));

This an excerpt of the app.xaml file:

<Application.Resources>
    <LinearGradientBrush x:Key="HeaderBackground" StartPoint="0.5,0" EndPoint="0.5,1">
        <GradientStop Color="#82cb02" Offset="1"/>
        <GradientStop Color="#82cb01" Offset="0.2"/>
        <GradientStop Color="#629a01" Offset="0.5"/>
    </LinearGradientBrush>
</Application.Resources>

What is the best way to allow this form of customizability (basically just changing some colors) to an application?

[Update]

I just found this answer from a previous question that was asked, and tried it but I am getting the same InvalidOperationException exception Petoj mentioned in the comments for the given answer. Here is the sample code from the answer:

Xaml:

<LinearGradientBrush x:Key="MainBrush" StartPoint="0,0.5" EndPoint="1,0.5" >
    <GradientBrush.GradientStops>
        <GradientStop Color="Blue" Offset="0" />
        <GradientStop Color="Black" Offset="1" />
    </GradientBrush.Gradie
wpf xaml resources customization app.xaml
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