add a custum.css file to a theme

I don’t know if the title here is right but here it goes
i am just trying to add some buttons to my theme ( Twentyten ) navbar
when i add the button css code in the style.css everything works fine
but i have a lot of buttons so i just want to create a separate css file just for that

When i create a custom link in the menu i do something like this
<a href="#">Home</a>

and add the button css code to style.css

.myButton {
	-moz-box-shadow: 0px 10px 14px -7px #3e7327;
	-webkit-box-shadow: 0px 10px 14px -7px #3e7327;
	box-shadow: 0px 10px 14px -7px #3e7327;
	background:-webkit-gradient(linear, left top, left bottom, color-stop(0.05, #77b55a), color-stop(1, #72b352));
	background:-moz-linear-gradient(top, #77b55a 5%, #72b352 100%);
	background:-webkit-linear-gradient(top, #77b55a 5%, #72b352 100%);
	background:-o-linear-gradient(top, #77b55a 5%, #72b352 100%);
	background:-ms-linear-gradient(top, #77b55a 5%, #72b352 100%);
	background:linear-gradient(to bottom, #77b55a 5%, #72b352 100%);
	filter:progid:DXImageTransform.Microsoft.gradient(startColorstr='#77b55a', endColorstr='#72b352',GradientType=0);
	background-color:#77b55a;
	-moz-border-radius:4px;
	-webkit-border-radius:4px;
	border-radius:4px;
	border:1px solid #4b8f29;
	display:inline-block;
	cursor:pointer;
	color:#ffffff;
	font-family:Arial;
	font-size:13px;
	font-weight:bold;
	padding:6px 12px;
	text-decoration:none;
	text-shadow:0px 1px 0px #5b8a3c;
}
.myButton:hover {
	background:-webkit-gradient(linear, left top, left bottom, color-stop(0.05, #72b352), color-stop(1, #77b55a));
	background:-moz-linear-gradient(top, #72b352 5%, #77b55a 100%);
	background:-webkit-linear-gradient(top, #72b352 5%, #77b55a 100%);
	background:-o-linear-gradient(top, #72b352 5%, #77b55a 100%);
	background:-ms-linear-gradient(top, #72b352 5%, #77b55a 100%);
	background:linear-gradient(to bottom, #72b352 5%, #77b55a 100%);
	filter:progid:DXImageTransform.Microsoft.gradient(startColorstr='#72b352', endColorstr='#77b55a',GradientType=0);
	background-color:#72b352;
}
.myButton:active {
	position:relative;
	top:1px;
}

so i am trying to say how can i point this button code to custom.css located in the wp-content/themes/twentyten