我知道这应该很容易，但我不知道我做错了什么。我只想用 Font Awesome 图标替换 png 背景图像（css）。
这是整个 css 文件：
.wc-bookings-date-picker .ui-datepicker{padding:0;width:auto;}
.wc-bookings-date-picker .ui-widget-content{border:0;background:#fff;-moz-box-shadow:0 1px 3px rgba(0,0,0,.19);-webkit-box-shadow:0 1px 3px rgba(0,0,0,.19);box-shadow:0 1px 3px rgba(0,0,0,.19);}
.wc-bookings-date-picker .ui-datepicker-header{padding:0;overflow:hidden;text-transform:uppercase;border-radius:4px 4px 0 0;-webkit-border-radius:4px 4px 0 0;-moz-border-radius:4px 4px 0 0;background-color:#999;background-image:none;background-image:-moz-linear-gradient(bottom,#999 0,#aaa 100%);background-image:-o-linear-gradient(bottom,#999 0,#aaa 100%);background-image:-webkit-linear-gradient(bottom,#999 0,#aaa 100%);background-image:linear-gradient(bottom,#999 0,#aaa 100%);border:0;border-top:1px solid #999;color:#fff;}
.wc-bookings-date-picker .ui-datepicker-header .ui-state-hover{background:0 0;border-color:transparent;cursor:pointer;}
.wc-bookings-date-picker .ui-datepicker .ui-datepicker-title{line-height:1em;font-weight:400;padding:.75em 0;}
.wc-bookings-date-picker .ui-datepicker .ui-datepicker-next,.wc-bookings-date-picker .ui-datepicker .ui-datepicker-prev{top:-1px;background:#999;border-radius:0;font-weight:400;height:2.5em;width:2em;}
.wc-bookings-date-picker .ui-datepicker .ui-datepicker-next.ui-state-hover,.wc-bookings-date-picker .ui-datepicker .ui-datepicker-prev.ui-state-hover{background:#aaa;}
.wc-bookings-date-picker .ui-datepicker .ui-datepicker-prev{left:0;border:0;}
.wc-bookings-date-picker .ui-datepicker .ui-datepicker-next{right:0;border:0;}
.wc-bookings-date-picker .ui-datepicker .ui-datepicker-next span,.wc-bookings-date-picker .ui-datepicker .ui-datepicker-prev span{margin-top:-4px;top:40%;}
.wc-bookings-date-picker .ui-icon-circle-triangle-e{background-image:url(../images/ui-icons_ffffff_256x240.png);background-position:-32px 0;}
.wc-bookings-date-picker .ui-icon-circle-triangle-w{background-image:url(../images/ui-icons_ffffff_256x240.png);background-position:-96px 0;} …