WhiteSur-gtk-theme/src/sass/gnome-shell/widgets-3-28/_dash.scss

#dash {
  color: $light_alt_fg_color;
  background-color: $light_divider_color;
  border-left: 0;
  border-radius: 0 ($bd_radius * 1.5) ($bd_radius * 1.5) 0;
  padding: 6px;

  &:rtl {
    border-radius: ($bd_radius * 1.5)  0 0 ($bd_radius * 1.5);
  }

  .placeholder {
    background-image: url("assets/dash-placeholder.svg");
    background-size: contain;
    height: 24px;
  }
}

.dash-item-container > StWidget {
  padding: 3px 6px;
}