drupal/core/modules/block/css/block.admin.css

#blocks tr.region-title td {
  font-weight: bold;
}
#blocks tr.region-message {
  font-weight: normal;
  color: #999;
}
#blocks tr.region-populated {
  display: none;
}
.block-region {
  background-color: #ff6;
  margin-top: 4px;
  margin-bottom: 4px;
  padding: 3px;
}
a.block-demo-backlink,
a.block-demo-backlink:link,
a.block-demo-backlink:visited {
  background-color: #b4d7f0;
  border-radius: 0 0 10px 10px;
  color: #000;
  font-family: "Lucida Grande", Verdana, sans-serif;
  font-size: small;
  line-height: 20px;
  left: 20px; /*LTR*/
  padding: 5px 10px;
  position: fixed;
  z-index: 499;
}
a.block-demo-backlink:hover {
  text-decoration: underline;
}

.layout-region {
  -webkit-box-sizing: border-box;
  -moz-box-sizing: border-box;
  box-sizing: border-box;
}
.block-list-secondary {
  border: 1px solid #bfbfbf;
  border-bottom-width: 0;
}
.block-list {
  padding: 0 0.75em;
  margin: 0;
}
.block-list li {
  list-style: none;
  padding: 0.1em 0;
}
.block-list a:before {
  content: '+ ';
}
.block-list-secondary .form-type-search {
  padding: 0 1em;
}
#block-placed {
  background-color: #ffd;
}

/* Wide screens */
@media
screen and (min-width: 780px),
(orientation: landscape) and (min-device-height: 780px) {

  .block-list-primary {
    float: left; /* LTR */
    width: 75%;
    padding-right: 2em;
  }
  [dir="rtl"] .block-list-primary {
    float: right;
    padding-left: 2em;
    padding-right: 0;
  }

  .block-list-secondary {
    float: right; /* LTR */
    width: 25%;
  }
  [dir="rtl"] .block-list-secondary {
    float: left;
  }

  /* @todo File an issue to add a standard class to all text-like inputs */
  .block-list-secondary .form-autocomplete,
  .block-list-secondary .form-text,
  .block-list-secondary .form-tel,
  .block-list-secondary .form-email,
  .block-list-secondary .form-url,
  .block-list-secondary .form-search,
  .block-list-secondary .form-number,
  .block-list-secondary .form-color,
  .block-list-secondary textarea {
    -webkit-box-sizing: border-box;
    -moz-box-sizing:    border-box;
    box-sizing:         border-box;
    width: 100%;
    max-width: 100%;
  }
}

/**
 * The vertical toolbar mode gets triggered for narrow screens, which throws off
 * the intent of media queries written for the viewport width. When the vertical
 * toolbar is on, we need to suppress layout for the original media width + the
 * toolbar width (240px). In this case, 240px + 780px.
 */
@media
screen and (max-width: 1020px),
(orientation: landscape) and (max-device-height: 1020px) {

  .toolbar-vertical .block-list-primary,
  .toolbar-vertical .block-list-secondary {
    float: none;
    width: auto;
    padding-right: 0;
  }
}