cesium/Source/Widgets/FullscreenButton/FullscreenButton.css

CesiumJS is a JavaScript library for creating 3D globes and 2D maps in a web browser without a plugin.

.cesium-button.cesium-fullscreenButton {
  display: block;
  width: 100%;
  height: 100%;
  margin: 0;
  border-radius: 0;
}