platform_largeScreen/public/static/cesium/Widgets/FullscreenButton/FullscreenButton.css

/* packages/widgets/Source/FullscreenButton/FullscreenButton.css */
.cesium-button.cesium-fullscreenButton {
  display: block;
  width: 100%;
  height: 100%;
  margin: 0;
  border-radius: 0;
}