yammer.css

@-moz-document url-prefix("https://www.yammer.com/") {

  div.page-content.two-column-layout,
  .column-two-left,
  .yj-main-content{
    margin-left: auto !important;
    margin-right: auto !important;
    width: 500px !important;
  }

  #column-two {
      width: 100% !important;
  }

  body,
  div.page-content.two-column-layout,
  div.page-content.three-column-layout {
    background: none !important;
    width: 100% !important;
  }

  #main-column {
    margin: 0 20px !important;
  }

  body #header-navigation,
  body #header-navigation #header-left,
  body #header-navigation #header-right {
      margin: 0 !important;
      width: auto !important;
  }

  #masthead-container,
  #column-one,
  #column-three,
  .yj-selector-right-sidebar,
  .yj-chatmanager,
  #footer {
    display: none !important;
  }

}