gnome-shell/data/theme/gnome-shell-sass/widgets/_workspace-switcher.scss

/* Workspace Switcher */
.workspace-switcher-group {
  padding: $base_padding * 2;
}

.workspace-switcher-container {
  @extend %osd_panel;
}

.workspace-switcher {
  background: transparent;
  border: none;
  border-radius: 0;
  padding: 0;
  spacing: $base_spacing * 2;
}

.ws-switcher-box {
  // background: transparent;
  background: transparent;
  height: 50px;
  background-size: 32px;
  border: 1px solid transparentize($osd_fg_color,0.9);
  border-radius: $base_border_radius + 3px;
}

// active workspace in the switcher
.ws-switcher-active-up,
.ws-switcher-active-down,
.ws-switcher-active-left,
.ws-switcher-active-right {
  background-color: $selected_bg_color;
  border: 1px solid if($variant=='light', darken($selected_bg_color, 8%), lighten($selected_bg_color, 5%));
  color: $selected_fg_color;
}

/* Workspace pager */

// thumbnails in overview
.workspace-thumbnails { 
  @extend %overview_panel;
  visible-width: 32px; //amount visible before hover
  spacing: $base_spacing;
  padding: $base_padding;

  border-radius: $modal_radius 0 0 $modal_radius;
  border-right-width: 0 !important;
  //fixme: can't have non uniform borders :(
  border-top-left-radius:0 !important;
  border-bottom-left-radius:0 !important;

  &:rtl { 
    border-radius: 0 $modal_radius $modal_radius 0; 
    border-left-width: 0 !important;
  }

  // drag and drop indicator
  .placeholder {
    background-image: url("resource:///org/gnome/shell/theme/dash-placeholder.svg");
    background-size: contain;
    height: 24px;
  }
}

// selected indicator
.workspace-thumbnail-indicator {
  border: 3px solid $selected_bg_color;
  border-radius: 3px;
  padding: 0px;
  // background-color: transparentize($selected_bg_color, 0.9);
}