While Blend makes it easy for you to create your own controls, creating your own control may not always be the best solution. In many cases, especially when you want to change how a control looks, styling an existing control may be a better choice. While you can style any control by using Blend's UI, modifying system controls can be time consuming if you are new to WPF and not sure what the consequences of modifying a particular property may be. To make it easier on you, Blend includes a resource dictionary called Simple Styles.xaml that applies an easily-modifiable style to your commonly used system controls.
To use these simple style controls, go to your Asset Library and select Simple Styles under the Controls tab:
You will see the simplified versions of your system controls, and they are just like your system controls...except much easier to style.
The User Guide provides great coverage of how to use simple style controls, so if you need any further information, hit F1 or go to Help | User Guide. In the User Guide, you can find Simple Styles listed under Home | Controls | Working with simple styles, or you can do what I did and just search the User Guide for "simple styles" instead!