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

.window-picker { //container around window thumbnails
  -horizontal-spacing: 16px;
  -vertical-spacing: 16px;
  padding: 0 16px 32px;
  spacing: 12px;

  &.external-monitor { padding: 16px; }
}

.window-caption {
  spacing: 25px;
  color: $light_alt_fg_color;
  background-color: $dark_bg_color;
  border-radius: $bt_radius;
  padding: 4px 8px;
  border: none;
  font-weight: normal;
  @include font(body);
}

.window-clone-border {
  border: 4px solid $light_track_color;
  border-radius: $bt_radius;
  // For window decorations with round corners we can't match
  // the exact shape when the window is scaled. So apply a shadow
  // to fix that case
  box-shadow: inset 0 0 0 1px $light_track_color;
}

$_close_size: 42px;

//close buttons
.window-close {
  background-size: $_close_size;
  height: $_close_size;
  width: $_close_size;
  -shell-close-overlap: 10px;
  border-radius: 100px;
  border: 0 none transparent;
  background-color: transparent;
  color: transparent;
  box-shadow: none;
  StIcon { icon-size: $_close_size; } // for safety
  background-image: url("assets/window-close.svg");

  &:hover {
    background-size: $_close_size;
    height: $_close_size;
    width: $_close_size;
    background-image: url("assets/window-close-hover.svg");
  }

  &:active {
    background-size: $_close_size;
    height: $_close_size;
    width: $_close_size;
    background-image: url("assets/window-close-active.svg");
  }
}