gnome-shell/data/theme/gnome-shell-sass/widgets/_osd.scss

/* OSD */

$osd_levelbar_height:6px;

.osd-window {
  @extend %osd_panel;
  text-align: center;
  font-weight: bold;
  &, & > * { spacing: $base_spacing * 2; } // 12px
  margin: 4em;

  StIcon {
    icon-size:$base_icon_size * 2;

    &:first-child:last-child { padding: 0 $base_padding * 2; }
  }

  .osd-monitor-label { font-size: 3em; }
  
  .level {
    height: $osd_levelbar_height;
    min-width:$base_icon_size * 10;
    -barlevel-height: $osd_levelbar_height;
    -barlevel-background-color: transparentize($osd_fg_color, if($variant=='light', 0.7, 0.9));
    -barlevel-active-background-color: $osd_fg_color;
    -barlevel-overdrive-color: $destructive_color;
    -barlevel-overdrive-separator-width: 3px;
    &:ltr {margin-right: 16px;}
    &:rtl {margin-left: 16px;}
  }
}

/* Pad OSD */
.pad-osd-window {
  padding: 32px;
  background-color: transparentize(#000, 0.2);

  .pad-osd-title-box { spacing: 12px; }
  .pad-osd-title-menu-box { spacing: 6px; }
}

.combo-box-label {
  width: 15em;
}

.resize-popup {
  @extend %osd_panel;
}