#back-top{position:fixed;left:50%;bottom:110px;display:none;width:50px;height:50px;margin-left:624px;color:#fff;font-size:36px;text-align:center;line-height:50px;border-radius:3px;background-color:rgba(0,0,0,.8);cursor:pointer}@media screen and (max-width:1440px){#back-top{margin-left:524px}}@media screen and (max-width:1080px){#back-top{margin-left:424px}}