You can define the look and feel of the WPF elements by setting properties such as FontSize and Background with the Button element as shown:
<Button Name="button1" Width="150" FontSize="12" Background="AliceBlue"> Click Me! </Button>
Instead of defining the look and feel with every element, you can define styles that are stored with resources. For completely customizing the look for controls, you can use templates and store them into resources.