怎么换门户背景图

/* ----------------------------------
Self-adaption Width CSS file for Discuz! X
(C) acgi.vip
http://acgi.vip
Created & Modified by OOZ.
---------------------------------- */
@charset "gbk";
*{padding:0; margin:0;}
li{list-style:none;}
img{border:0;}
a{text-decoration:none;}
#toptb{font-family:'SimSun', 'Helvetica Neue',sans-serif,Helvetica,Arial;}
#toptb a{color:#999;}
/*DIY*/
#diy-tg{background:none; float:none;display:block;width:100%;padding:0 15px!important;text-indent:0px; }
#diy-tg:hover,.acgimessage ul a:hover{background:#00cc66; color:#fff;}
.topdiy{width:50px;display:inline-block;overflow:hidden;}
/*common*/
.acgiborder{border:1px solid #f0f0f0!important;}
.acgifff{background:#fff;}
.top15{margin-top:15px!important;}
.top25{margin-top:20px!important;}
.clear{clear:both;}
.top80{padding-top:50px!important;}
a:hover{text-decoration:none;}

.acgiduang{-webkit-transition: all 0.5s;-moz-transition: all 0.5s;-o-transition: all 0.5s;transition: all 0.5s;position: relative;}
.acgiduang:hover{-webkit-animation-name: jello;animation-name: jello;-webkit-animation-duration: 1s;animation-duration: 1s;-webkit-animation-fill-mode: both;animation-fill-mode: both;}
.acgixz{transition:All 0.4s ease-in-out; -webkit-transition:All 0.4s ease-in-out; -moz-transition:All 0.4s ease-in-out; -o-transition:All 0.4s ease-in-out;}
.acgixz a:hover{transform:rotate(360deg); -webkit-transform:rotate(360deg); -moz-transform:rotate(360deg); -o-transform:rotate(360deg); -ms-transform:rotate(360deg);}
.acgimove:hover{transform: translate(10px,0);-webkit-transform: translate(10px,0);-moz-transform: translate(10px,0);-o-transform: translate(10px,0);-ms-transform: translate(10px,0);}
.acgimove{transition: All 0.4s ease-in-out;-webkit-transition: All 0.4s ease-in-out;-moz-transition: All 0.4s ease-in-out;-o-transition: All 0.4s ease-in-out;}
.acgilarge a img{transition: all 0.6s; -moz-transition: all 0.6s; -webkit-transition: all 0.6s; -o-transition: all 0.6s;}
.acgilarge a:hover img{transform: scale(1.2); -moz-transform: scale(1.2); -webkit-transform: scale(1.2); -o-transform: scale(1.2); -ms-transform: scale(1.2);}
.acgizhendong:hover{-webkit-animation:icon-bounce 0.7s alternate;-moz-animation:icon-bounce 0.7s alternate;-o-animation:icon-bounce 0.7s alternate;animation:icon-bounce 0.7s alternate;}
.acgixx:hover{filter: alpha(opacity=0);-webkit-transition: all .3s ease;-moz-transition: all .3s ease;-o-transition: all .3s ease; transition: all .3s ease;}
html{min-width:1160px;/*background:url() no-repeat 50% 0 fixed*/;}
/*body*/
body{min-height:600px;font-family: "Arial","Microsoft YaHei","黑体","宋体",sans-serif;/*font-family:'SimSun', 'Helvetica Neue',sans-serif,Helvetica,Arial;*/background: linear-gradient(rgba(255,255,255,0),#f9f9f9 1070px,#f9f9f9 100%),rgba(255,255,255,0.6);}

html,.acgihead{background:url() no-repeat;}
/**//*网站流量有压力就换新浪图片 效果一样*/
html{background-position: 50% -195px;}

.acgihead{height: 170px; width: 100%;background-position:50% -276px;}
.acgiheadmask{height:100%;width:100%;background: url({STYLEIMGDIR}/white_40.png);}/*横幅透明度 可选 20 40 60 80 黑色换前缀 black_*/
.acgiwp,.in-wp,.wp{width:960px; margin:0 auto;}
.acgi1080{width:1080px; margin:0 auto;position:relative;}
.acgi1120{width:1120px; margin:0 auto;}
.acgiwp{height:100%;}
.acgiwp a.logo{display:inline-block;}
.in-wp{background:url