cong-gis-viewer/dist/cesiumStatic/Widgets/FullscreenButton/FullscreenButton.css

The minimal recommended setup for an app using Cesium with Webpack 5

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