If you have in your WPF project an ResourceDictionary that you use to define resources you can create an instance of it from code like this:

ResourceDictionary res = Application.LoadComponent(
 new Uri("/WpfApplication7;component/RedRibbonBar.xaml", 
 UriKind.RelativeOrAbsolute)) as ResourceDictionary;

Where WpfApplication7 is name of your assembly and RedRibbonBar.xaml is name of your ResourceDictionary.

Another less known way of doing same thing is defining the code-behind for the resource dictionary. I have not found an automated way of doing this in VS.NET 2010 so I do it manually.

First you add x:Class to your ResourceDictionary definition like so:

<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
 x:Class ="WpfApplication7.RedRibbonBar">

Then you add class to your project that holds code-behind and you name it RedRibbonBar.xaml.cs where you change the name to whatever your resource dictionary name is.

The code behind class looks like so:

partial class RedRibbonBar: ResourceDictionary
{
   public RedRibbonBar()
   {
      InitializeComponent();
   }     
}

Note that you need to change RedRibbonBar I used here to the name of your ResourceDictionary. Then you create simply new instance of your dictionary like so:

ResourceDictionary res = new RedRibbonBar();