You have the right idea, but you are setting the Foreground property of the wrong element and also you'll need to take advantage of the new implicit theming ability for the controls. You want to aim for the part that contains the text that you want to change the foreground of, just applying a Foreground property to the entire Item template won't work. Especially since you are using the style manger, it will override any set style you prepare.
Go to this link
first, and learn about implicit styles. This is what you will need to do if you want to use the metro them, yet only change the foreground on a TreeViewItem. Take your time on this section and learn how to set it up properly, once you do you'll have a great amount of control over the appearance of your application.
Now that you've read up on implicit styles, lets take a look at this link
, it will bring you to the RadTreeView template structure. It shows you all of the parts of the control template. That will give you an idea of where to begin when altering the template.
Next, lets look at the TreeViewItemTemplate. Find that at this link
. The text is inside a ContentPresenter named Header
which is bound to your theme via TemplateBinding to the Header
item in your theme. I've attached an image to this post to show you where to look.
Once you do this, any generated items will match your Foreground color in any part of the app you have set your theme to. This is a couple extra steps than you would normally do, but once accomplished provide great flexibility in your application.