Do be aware that windows, dialogs, message boxes, and popups are the root of their own visual tree, and as such they do not "inherit" properties that are set on the UI element that owns them. In particular, this means that if you want to have uniform usage of fonts throughout your application, you will have to set the font family, font size, etc. for each window, dialog, message box, or popup. The same goes for other window related properties such as the cursor. Failure to do so will mean that your custom window uses the values that are provided by the Microsoft default Microsoft styles.

The following example illustrates how to define a new custom window in XAML and set the font to 15pt Philips Healthcare Font, which is defined as a resource elsewhere.

  ✔  <Window x:Class="MyCustomWindow"
    FontFamily="{Window PhilipsHealthcareFont}"
    FontSize="14"/>