Take a look at this very detailed post on styling
Silverlight controls.
Source: The Microsoft Expression Blog
Microsoft Expression Blend comes with many Microsoft Silverlight controls that you can use to create a great user experience. You can also download controls from trusted sources.
For more information, see Import a custom control by adding a reference.
If you find a control that functionally fits your needs but doesn't look the way that you want it to, and if that control supports templates, you can modify the templates of the control to change its appearance.