gnome-shell/data/theme/gnome-shell-sass/widgets/_window-picker.scss

/* Window Picker */

$window_picker_spacing: $base_spacing; // 6px
$window_picker_padding: $base_padding * 2; // 12px

$window_close_button_size: 24px;
$window_close_button_padding: 3px;

// Window picker
.window-picker {
  // Space between window thumbnails
  spacing: $window_picker_spacing;
}

// Window titles
.window-caption {
  color: $osd_fg_color;
  background-color: $osd_bg_color;
  border:1px solid $osd_outer_borders_color;
  border-radius: $base_border_radius + 1;
  padding: $base_padding $base_padding * 2;
  font-weight: bold;
  @include fontsize($base_font_size + 1);
}

// Close button
.window-close {
  background-color: $selected_bg_color;
  color: $selected_fg_color;
  border: none;
  border-radius: $window_close_button_size * 0.5 + $window_close_button_padding * 2;
  padding: $window_close_button_padding;
  height: $window_close_button_size;
  width: $window_close_button_size;
  box-shadow: -1px 1px 5px 0px rgba(0,0,0,0.5);

  &:hover {
    background-color: lighten($selected_bg_color, 5%);
  }

  &:active {
    background-color: darken($selected_bg_color, 5%);
  }
}