How to define and use resources in xaml so they can be used in C#

Theoretically, I think that I can define Brushes and Colors etc. in an xaml file and assign that to a button.background in c#. But how do I do that? Where do I put my lineargradientbrush definition like this:

<LinearGradientBrush x:Key="BlaBrush">
                <GradientStop Offset="0" Color="Red"/>
                <GradientStop Offset="1" Color="Green"/>
</LinearGradientBrush>

Just putting it at various places in my window's xaml file results in various error messages :/

I found this question here on stackoverflow: How to use a defined brush resource in XAML, from C# which explains a part of it, but he seems to know where to do the Brush definition.

I also tried adding the shinyblue.xaml wpf template to the app and added <ResourceDictionary Source="ShinyBlue.xaml"/> to the application.resources in app.xaml. This makes all my buttons blue instantly, but still, the "things" defined in shinyblue.xaml like NormalBrush is not accessible from C# - at least I don't know how.

Marc