@media screen and (max-width: 1024px) {
    #main {
        width: 100%;
        margin: 0;
        padding: 0;
    }

    #header {
        width: 100%;
        margin: 0;
        padding: 0;
    }
}

#header {
    background-image: url(../images/header.jpg);
    width: 53%;
    height: 239px;
    background-repeat: no-repeat;
    padding: 0px;
    margin: auto;
    border: 1px solid #CCC;
}

#main {
    width: 53%;
    min-height: 800px;
    height: auto;
    margin: auto;
    background-color: #F5F5F5;
    border: 1px solid #CCC;
    overflow: auto;
    font-family: serif;
}

#main {
  width: 53%;
}

@media screen and (max-width: 1024px) {
  #main {
    width: 100%;
  }
}

@media screen and (max-width: 1024px) {
  #main {
    width: 100%;
  }
}

@media screen and (min-width: 1025px) {
  #main {
    width: 100%;
  }
}

@media screen and (max-width: 1024px) {
...
}
@media not screen and (max-width: 1024px) {
...
}