How to change the background color of messagestack

I would like to change the background color of messagestack from red, green, yellow to a different color to match the color scheme of my site.

Pls tell me what files do I need to modify. I tried looking up for class for eg messageStackError , 'class="messageStackSuccess larger" etc as specified in the message_stack.php but could not find these classes.

thanks in advance for your help

in stylesheet.css

.messageStackWarning, .messageStackError {
	background-color: #ff0000;
	color: #ffffff;
	}

.messageStackSuccess {
	background-color: #99FF99;
	color: #000000;
	}

.messageStackCaution {
	background-color: #FFFF66;
	color: #000000;
	}

thanks for quick response...it works!!