Remove background on hover for menu item

i’m really struggling to remove the background for the menu items when they are hovered over. i had to go into the core css style sheet rather than use my child theme and it seemed to work. however i’ve just updated the responsive theme and it’s lost and when i go to do the same thing again it doesn’t work. i thought it was this part of the stylesheet:

.menu a:hover {
	background-color: #808080;
	background-image: -webkit-gradient(linear, left top, left bottom, from(#808080), to(#363636));
	background-image: -webkit-linear-gradient(top, #808080, #363636);
	background-image: -moz-linear-gradient(top, #808080, #363636);
	background-image: -ms-linear-gradient(top, #808080, #363636);
	background-image: -o-linear-gradient(top, #808080, #363636);
	background-image: linear-gradient(top, #808080, #363636);
	color: #ffffff;
	filter: progid:DXImageTransform.Microsoft.gradient(startColorstr=#808080, endColorstr=#363636);
}

but it doesn’t seem to do anything if i change it. any advice much appreciated, probably something really simple but have tried all i can think of!