WhiteSur-gtk-theme/src/sass/gnome-shell/common/_search-entry.scss

// Search entry
%search-entry, .search-entry {
  width: 320px;
  min-height: $menuitem_size;
  padding: 2px 10px;
  border: none;
  color: $light_hint_fg_color;
  caret-color: $light_alt_fg_color;
  selection-background-color: $light_track_color;
  selected-color: $light_alt_fg_color;
  @include search_entry(normal, $fc:$light_track_color);

  &:focus {
    @include search_entry(focus, $fc:$light_alt_fg_color);
    border: none;
    color: $light_alt_fg_color;
  }

  .search-entry-icon {
    icon-size: 16px;
    padding: 0 0;
    color: $light_hint_fg_color;
  }

  &:hover, &:focus {
    .search-entry-icon { color: $light_alt_fg_color; }
  }

  &:insensitive {
    border: none;
    color: $light_disabled_fg_color;
  }

  StLabel.hint-text {
    color: $light_hint_fg_color;
  }
}