Custom css to override Plugin Css

Hi, I am trying to turn this code from the box-shortcodes.css:

/* Styles for screens that are less than 768px */
@media only screen and (max-width: 768px) {
	.su-column {
		width: 100% !important;
		margin: 0 0 1.5em 0 !important;
		float: none !important;
	}
	.su-row .su-column:last-child {
		margin-bottom: 0 !important;
	}
}

into this:

/* Styles for screens that are less than 768px */
@media only screen and (max-width: 768px) {
	.su-column {
		margin: 0 0 1.5em 0 !important;
		float: none !important;
                font-size: 12px;
	}
	.su-row .su-column:last-child {
		margin-bottom: 0 !important;
	}
}

or just remove that media query altogether. When I make the changes in the “Custom Css,” nothing changes. I may be doing something wrong…any help?

https://wordpress.org/plugins/shortcodes-ultimate/