a {
    float: left;
    width: 32px;
    height: 32px;
    text-align: left;
    text-indent:-9999px;
    background: url('../img/button.png') no-repeat 0 0;
    -webkit-transition: background 300ms ease-in 2s; /* property duration timing-function delay */
    -moz-transition: background 300ms ease-in 2s;
    -o-transition: background 300ms ease-in 2s;
    transition: background 300ms ease-in 2s;
    -webkit-transition-property: background;
    -webkit-transition-duration: 300ms;
    -webkit-transition-timing-function: ease-in;
    -webkit-transition-delay: 100ms;
    -moz-transition-property: background;
    -moz-transition-duration: 300ms;
    -moz-transition-timing-function: ease-in;
    -moz-transition-delay: 100ms;
    -o-transition-property: background;
    -o-transition-duration: 300ms;
    -o-transition-timing-function: ease-in;
    -o-transition-delay: 100ms;
    transition-property: background;
    transition-duration: 300ms;
    transition-timing-function: ease-in;
    transition-delay: 100ms;
}
a:hover {
    background:position: 0 -32px;
}
..現在はスクロールアップ/ダウン効果がありますが、フェード効果で背景を変更したいのですが、CSSで何を変更すればよいですか?
ありがとう!