body {
  background: #202124 url(https://files-cdn.cnblogs.com/files/qpone/2e5cda9ec596cc2f6bd74d5c1169d184.bmp) no-repeat center fixed;
background-size:cover;
margin : 0 auto;
}
#main{
opacity : 0.8;
}
#sideBar{
background-color : lightblue;
}