There are two possibilities at present. The first is to edit one of the Styles included with the plugin. I do NOT recommend this at present as it will be overwritten when the plugin is next upgraded -- though I hope to fix this in a future update.
The best way to do this is to put the relevant CSS directly into your Theme. In the theme folder, add a file called "jspullquotes.css" and copy the relevant CSS into that file. Then go into Pull-Quotes settings in WordPress and check the "Do not link CSS" box and save settings.
To see how the existing pull-quote styles work, manually download the plugin and look at the contents of the Styles directory. Note also (this is important!) that you also should copy over the contents of "/resources/jspullquotes-core.css". Copy the core stuff into your jspullquotes.css file, and then copy the Style-specific stuff over as well.
Again, I hope to make this a bit more "newbie friendly" in a future update.