Button display

Hi,
I am using a button from ‘optin monster’ that displays with an unwanted grey box in the background. I did some research in their support area, and this is what they said:

Very similar implementation to the text link, but this time you use it to make a button. Your default button design depends on your CSS on your site.
Click me!
You may need to replace the class “button” with your own theme-specific class. Some themes use button as a class while others use alternative naming conventions. Refer to your theme’s documentation for information about button classes.

Does this seem right, and if so can you tell me what the class name should be (and then also, where do I change it) ?

Thanks very much
Richard