How do I modify color used in Flat calendar style?

I used Firefox and Firebug, and found where the color is set, but I'm not sure where and how to make the changes so I can modify the colors. I'm using the Flat calendar style, and the color is red, not complimentary to the color scheme of the site. You can view my site's calendar here: JEvents calendar for dev site

I copied the CSS (see below) I found using Firebug, modified it for #c28c51, and then pasted it in JEvents Calendar Custom CSS editor. But nothing changed.

div.nextmonth {
background-color: #ff3b30;
border: 1px solid #ff3b30;
}

Where did I go wrong? Thanks for any help.

Pat