Easy Digital Downloads comes with some default CSS styling. At the top of the Styles Settings page you may choose to turn these off to avoid problems with your own styles.
The next option is to set a default of either Plain Text Button, or a traditional Button. The image to the right has a Plain Text Button at the top.
Lastly you may choose a default button color. To the right is a rendering of all of the colors. "Inherit" has no color set by EDD, and can be customized by the theme.
Checkout Style Comparisons
Here is the checkout page with default styles as well as with styles turned off. Click the thumbnails for full versions.