Goodreads widget background edit

I am using the Jetpack Goodreads widget in a sliding side panel of my site. The theme (HighEnd by HB Themes) presents the side panel with a black background, which happens to work for my site. The Goodreads widget, however, renders a white background–with white text–which obviously won’t work. I found the css to change the Goodreads widget to black, and it works perfectly–until there is a Jetpack update (like today). I have copied the css–with the appropriate change–to my child-theme css file in an effort to override the changes that occur with an update, but it won’t work (perhaps because the Jetpack css is loaded after the theme’s?). I have turned on minification, so perhaps that’s part of the problem? Where would I tuck the customized css so that it controls the Goodreads background regardless of Jetpack updates? Here’s the code with my change to the background:

div[class^="gr_custom_container"] {
	/* customize your Goodreads widget container here*/
	border: 1px solid gray;
	border-radius:10px;
	padding: 10px 5px 10px 5px;
	background-color: #000;
	color: #000;
}