gnome-control-center/panels/background/preview.css

background-preview {
  border-radius: 6px;
}

background-preview .window {
  border-radius: 6px;
  box-shadow: 0 1px 4px 1px alpha(black, 0.13),
              0 1px 10px 5px alpha(black, 0.09),
              0 3px 16px 8px alpha(black, 0.04),
              0 0 0 1px alpha(black, .05);
}

background-preview .window .header-bar {
  min-height: 15px;
  box-shadow: inset 0 -1px @borders;
}

background-preview .window.light {
  background-color: #fafafa;
  color: alpha(black, .8);
}

background-preview .window.front.light .header-bar {
  background-color: #ebebeb;
}

background-preview .window.dark {
  background-color: #242424;
  color: white;
}

background-preview .window.front.dark .header-bar {
  background-color: #303030;
}

.background-preview-button {
  background: none;
  border-radius: 9px;
  padding: 3px;
  box-shadow: none;
  outline: none;
}

.background-preview-button:checked {
  box-shadow: 0 0 0 3px @accent_color;
}

.background-preview-button:focus:focus-visible {
  box-shadow: 0 0 0 3px alpha(@accent_color, .3);
}

.background-preview-button:checked:focus:focus-visible {
  box-shadow: 0 0 0 3px @accent_color, 0 0 0 6px alpha(@accent_color, .3);
}

.slideshow-icon {
  color: white;
  -gtk-icon-shadow: 0 1px 2px rgba(0, 0, 0, 0.33);
}

button.remove-button {
  border-radius: 9999px;
  padding: 1px 0px; /* circles instead of ellipses */
  background-origin: padding-box, border-box;
  background-clip: padding-box, border-box;
}