Custom CSS class isn't showing

I’m using a shortcode for a button in a text widget, on a site built on the Gantry framework. (You can see a page-in-progress here.) Since Gantry already provides styles for buttons, I wanted to give the CSS3 buttons the relevant class, which I believe is just .button, so it would pick up Gantry’s style.

Unfortunately, when I try to give the CSS3 button the class “button”, whether it’s via the shortcode (cssclass="button") or setting the default class via the settings menu, it doesn’t stick. When I load the page, the button reflects the default style I’ve selected from the drop-down in the settings page, and upon inspection of the HTML, I can see that the button doesn’t even have the “button” class I gave it.

If I use the shortcode to give the button a class defined by the plugin, such as .button-twitter or .button-rss, that works just fine; the button adopts the class and the styling associated with it. In short, it appears that this plugin only adds classes that are defined in standout-css3-buttons.css.

This is inconvenient, because I’d rather not duplicate CSS that already exists elsewhere in my theme. Additionally, if I have to edit standout-css3-buttons.css in order to add custom classes, I’m concerned that any time the plugin updates, I’ll lose my work and have to add them again. This is terribly inconvenient.

How can I use this widget to add custom classes to the buttons without having to specifically edit standout-css3-buttons.css?

Thank you.

https://www.ads-software.com/plugins/standout-css3-buttons/