atom/static/tabs.css

.tabs {
  font: caption;
  -webkit-user-select: none;
  display: -webkit-box;
  -webkit-box-align: center;
}

.tab {
  cursor: default;
  padding: 2px 21px 2px 9px;
  -webkit-box-flex: 2;
  position: relative;
  width: 175px;
  max-width: 175px;
  min-width: 40px;
  box-sizing: border-box;
  text-shadow: 0 -1px 1px black;
}

.tab.active {
  -webkit-box-flex: 1;
}

.tab.active:before,
.tab.active:after {
  position: absolute;
  bottom: -1px;
  width: 4px;
  height: 4px;
  content: " ";
  z-index: 3;
}

.tab.active:before {
  left: -4px;
}

.tab.active:after {
  right: -4px;
  border-width: 0 0 1px 1px;
}

.tab.active:first-child:before {
  display: none;
}

.tab .file-name {
  font-size: 11px;
  display: block;
  overflow: hidden;
  white-space: nowrap;
  text-overflow: ellipsis;
  padding: 3px 5px;
}

.tab .close-icon {
  font-family: 'Octicons Regular';
  font-size: 12px;
  width: 12px;
  height: 12px;
  cursor: pointer;
  position: absolute;
  right: 5px;
  top: 4px;
  -webkit-font-smoothing: antialiased;
}

.tab .close-icon:before {
  content: "\f081";
}

.tab .close-icon:hover {
  color: white;
}

.tab.file-modified .close-icon {
  top: 11px;
  width: 5px;
  height: 5px;
  right: 5px;
  border: 2px solid #fff;
  border-radius: 12px;
}

.tab.file-modified .close-icon:before {
  content: "";
}