以下のcssがあなたのニーズに合うと思います。
CSS :
#header {
width:1000px;
height:250px;
/* IE10 Consumer Preview */
background-image: -ms-linear-gradient(bottom, #EDD799 0%, #BF7F37 100%);
/* Mozilla Firefox */
background-image: -moz-linear-gradient(bottom, #EDD799 0%, #BF7F37 100%);
/* Opera */
background-image: -o-linear-gradient(bottom, #EDD799 0%, #BF7F37 100%);
/* Webkit (Safari/Chrome 10) */
background-image: -webkit-gradient(linear, left bottom, left top, color-stop(0, #EDD799), color-stop(1, #BF7F37));
/* Webkit (Chrome 11+) */
background-image: -webkit-linear-gradient(bottom, #EDD799 0%, #BF7F37 100%);
/* W3C Markup, IE10 Release Preview */
background-image: linear-gradient(to top, #EDD799 0%, #BF7F37 100%);
}
http://jsfiddle.net/xPLPH/
線形グラデーションの詳細:
https://developer.mozilla.org/en-US/docs/CSS/linear-gradient