Hi Adrian,
The styling rule in the widget is more specific than the custom one you specified, therefore the rule you added is ignored (you can see this because the rule gets striked through when inspecting). In cases like this, try to make your styling more specific, for example:
.widget-listview-swipe .mx-listview-item > .mx-dataview > .widget-listview-swipe-foreground {
background-color: #C4DFFC;
}
Hope this helps!