body {

display: flex;
min-height: 100vh;
flex-direction: column;
.flex-main {
  display: flex;
  flex: 1;
  .flex-fixed-column {  
    flex: 0 0 300px;
    order: -2;
  }
  .flex-seperator {
    flex: 0 0 1px;
    order: -1;
    background: #333;
    margin: 0 20px;
  }
  .flex-content {
    width: 100%;
    flex: 1;
  }
}

}