Hi,
That CSS is not added by us. It's inside the library notify-js which HikaShop uses to display the notification boxes.
You can find it in the file media/com_hikashop/js/notify.js with the code:
addStyle("metro-lite", {
		html: '<div><div class="text-wrapper"><div class="title" data-notify-html="title"></div><div class="text" data-notify-html="text"></div></div></div>',
		classes: {
			error: {
				"color": "#a94442 !important",
				"background-color": "#f2dede",
				"border": "1px solid #ebccd1"
			},
			success: {
				"color": "#3c763d !important",
				"background-color": "#dff0d8",
				"border": "1px solid #d6e9c6"
			},
			info: {
				"color": "#31708f !important",
				"background-color": "#d9edf7",
				"border": "1px solid #bce8f1"
			},
			warning: {
				"color": "#8a6d3b !important",
				"background-color": "#fcf8e3",
				"border": "1px solid #faebcc"
			},
			black: {
				"color": "#fafafa !important",
				"background-color": "#333",
				"border": "1px solid #000"
			},
			white: {
				"background-color": "#f1f1f1",
				"border": "1px solid #ddd"
			}
		}
	});It indeed cannot be overridden in CSS.
To override it, you would have to add that same javascript modified with the colors you want in a view file used where the notification is displayed.
That way, your javascript would override the "metro" style used by notify-js with your own colors.