gnome-shell/data/theme/gnome-shell-sass/widgets/_switcher-popup.scss

/* App Switcher */

// same as dash
$switcher_padding: $base_padding + 4px; // 10px
$switcher_border_radius: $modal_radius + 8px;


// the full screen container of the switcher
.switcher-popup {
  padding: 0;
  spacing: $base_padding * 4;
}

// switcher onscreen panel
.switcher-list {
  @extend %osd_panel;
  padding: $switcher_padding;
  border-radius: $switcher_border_radius;
  box-shadow: 0 8px 8px 0 rgba(0,0,0,0.1);

  // container for items in list
  .switcher-list-item-container {
    spacing: $base_padding * 2;
  }

  // each item in the list
  .item-box {

    @include tile_button($osd_fg_color);
  }

  .separator {
    width: 1px;
    background: $borders_color;
  }

  // container of thumbnails
  .thumbnail-box {
    padding: 2px;
    spacing: $base_padding;
  }

  // window thumbnail itself
  .thumbnail {
    width: 256px; // equal to THUMBNAIL_DEFAULT_SIZE in altTab.js
    border-radius:$base_border_radius;
  }
}

// arrow if app has multiple windows
.switcher-arrow {
  border-color: transparentize($osd_fg_color, 0.2);
  color: transparentize($osd_fg_color, 0.2);

  &:highlighted {
    border-color: $osd_fg_color;
    color: $osd_fg_color;
  }
}

// Input Source Switcher
.input-source-switcher-symbol {
  font-size: 34pt;
  width: 96px;
  height: 96px;
}

// Window cycler highlight
.cycler-highlight {
  border: 5px solid $selected_bg_color;
}