The Popup Settings panel let you determine the default popup orientation as well as provide your own CSS. By Clicking the "set basic CSS rules" link you will set the basic CSS rules in the text editor. These are the default settings rules. You can change values in order to change style and/or add your own code. The CSS code you will set in the text editor will be appended automatically to the page boxy when notification popup is presented. 

Note: Changing the CSS assume that you have basic understanding in styling HTML using CSS rules.