You can you create resource dictionaries to define default styles globally.
You can also reference a resource dictionary or define a style in any object.
In either case those styles will apply to all child objects that don't have a style explicitly defined...
Example:
<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" >
<!--Default styles that will apply to any object of the specified type (if it doesn't have style set locally)-->
<Style TargetType="Label" >
<Setter Property="FontWeight" Value="Bold"/>
<Setter Property="Margin" Value="0"/>
<Setter Property="Padding" Value="0"/>
</Style>
</ResourceDictionary>