Using simple styles is a great way to dig into template editing. You can learn a lot about customizing controls byt using them. There is great document within the help file that describes in detail how simple styles work, I would highly encourage people to read that.
After playing with simples styles for while, when you are learning and playing with simple styles in your project there is a point when you probably would want to get a clean start since you know have a good idea of what you want to do. The way simple styles work is whenever you instantiate a control from the simple style resource dictionary; Blend will make a copy of that resource dictionary locally in the project you are working on. If you want to go back to initial state of the simple styles resource dictionary there is a way to force an reset through the Asset Palette. In the picture below you can the a dropdown from the SimpleStyle item that has an option to Reset. This option will only show up after making an edit to a resource in the Resource Dictionary. After you reset the Resource Dictionary the file is copied over so you are back to the starting point, having in mind that you will loose all the edits that you did to SimpleStyles.xaml. Which in many cases is what you want to do.