[Ultimate shortcodes] How to use the class?

Hi there. I am liking the ultimate shortcodes plugin however on the “tabs” type I do not like that it reduced my font size to 13px.

I noticed you could set a class:
[su_tabs vertical=”yes” class=”testclass”]

However I am unsure where to place the css override AND what it should look like.

Original code:

.su-tabs-pane {
padding: 15px;
font-size: 13px;
-webkit-border-bottom-right-radius: 3px;
-moz-border-radius-bottomright: 3px;
border-bottom-right-radius: 3px;
-webkit-border-bottom-left-radius: 3px;
-moz-border-radius-bottomleft: 3px;
border-bottom-left-radius: 3px;
background: #fff;
color: #333;
}

So maybe.testclass.su-tabs-pane or .su-tabs-pane-testclass ?

Additional questions:
-Do I input it at Shortcodes>Custom CSS?

-Is there a way to just disable the fontsize and let the box just use the site default (that way I do not have to re-do the css changes when I change website font sizes).