layout.css

1. 9.x drupal/core/themes/bartik/css/layout.css
2. 9.x drupal/core/themes/stark/css/layout.css

/**
 * @file
 * Stark layout method
 *
 * To avoid obscuring CSS added to the page by Drupal or a contrib module, the
 * Stark theme itself has no styling, except just enough CSS to arrange the page
 * in a traditional "Header, sidebars, content, and footer" layout.
 *
 * This layout method works reasonably well, but shouldn't be used on a
 * production site because it can break. For example, if an over-large image
 * (one that is wider than 20% of the viewport) is in the left sidebar, the
 * image will overlap with the #content to the right.
 */

#content,
#sidebar-first,
#sidebar-second {
  display: inline;
  position: relative;
}

img {
  height: auto;
  max-width: 100%;
}

@media all and (min-width: 480px) and (max-width: 959px) {
  #content {
    width: 100%;
  }
  body.sidebar-first #content,
  body.two-sidebars #content {
    width: 67%;
    float: right; /* LTR */
  }
  body.sidebar-second #content {
    width: 67%;
    float: left; /* LTR */
  }

  #sidebar-first {
    width: 33%;
    float: left; /* LTR */
  }

  #sidebar-second {
    width: 33%;
    float: right; /* LTR */
  }
  body.two-sidebars #sidebar-second {
    clear: both;
    width: 100%;
  }

  body.two-sidebars #sidebar-second .block {
    float: left; /* LTR */
    width: 33%;
  }
  body.two-sidebars #sidebar-second .block:nth-child(3n+1) {
    clear: both;
  }
}

@media all and (min-width: 960px) {
  #content {
    width: 100%;
    float: left; /* LTR */
  }
  body.sidebar-first #content {
    width: 80%;
    left: 20%; /* LTR */
  }
  body.sidebar-second #content {
    width: 80%;
  }
  body.two-sidebars #content {
    width: 60%;
    left: 20%; /* LTR */
  }

  #sidebar-first {
    width: 20%;
    float: left; /* LTR */
    left: -80%; /* LTR */
  }
  body.two-sidebars #sidebar-first {
    left: -60%; /* LTR */
  }

  #sidebar-second {
    float: right; /* LTR */
    width: 20%;
  }
}

/**
 * Responsive tables.
 */
@media screen and (max-width:28.125em) { /* 450px */
  th.priority-low,
  td.priority-low,
  th.priority-medium,
  td.priority-medium {
    display: none;
  }
}
@media screen and (max-width:45em) { /* 720px */
  th.priority-low,
  td.priority-low {
    display: none;
  }
}