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

/* Window Picker */
$window_close_button_color: transparentize(lighten($system_bg_color, 7%), .02);

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

// Window icons
.window-icon {
  // styled only with .icon-dropshadow
  // but has a different style in high-contrast
  @if $contrast == 'high' {
    background-color: $osd_bg_color;
    padding: $base_padding * 2;
    border-radius: $modal_radius;
    border: 2px solid $hc_inset_color;
    margin: $base_padding * 3;
  }
}

// Window titles
.window-caption {
  @extend %tooltip;
}

// Close button
.window-close {
  background-color: $window_close_button_color;
  color: $system_fg_color;
  border-radius: $forced_circular_radius;
  box-shadow: 0 2px 4px 0 $shadow_color;
  padding: $base_padding * 0.5;
  height: $large_icon_size;
  width: $large_icon_size;
  transition-duration: 100ms;
  border: 2px solid transparent;

  @if $contrast == 'high' {
    border-color: $hc_inset_color;
  }

  & StIcon { icon-size: $medium_icon_size; }

  &:hover {
    background-color: lighten($window_close_button_color, 7%);
  }

  &:active {
    background-color: lighten($window_close_button_color, 13%);
  }

}

.workspace-background {
  // keep in sync with BACKGROUND_CORNER_RADIUS_PIXELS in workspace.js
  border-radius: 30px;
  background-color: $invisible_occluded_bg_color;
  box-shadow: 0 4px 16px 4px $shadow_color;
}