Source

File: wp-includes/class-wp-customize-manager.php

public function customize_preview_loading_style() {
		?>
		<style>
			body.wp-customizer-unloading {
				opacity: 0.25;
				cursor: progress !important;
				-webkit-transition: opacity 0.5s;
				transition: opacity 0.5s;
			}
			body.wp-customizer-unloading * {
				pointer-events: none !important;
			}
			form.customize-unpreviewable,
			form.customize-unpreviewable input,
			form.customize-unpreviewable select,
			form.customize-unpreviewable button,
			a.customize-unpreviewable,
			area.customize-unpreviewable {
				cursor: not-allowed !important;
			}
		</style>
		<?php
	}