Source

File: wp-admin/includes/class-wp-theme-install-list-table.php

public function theme_installer() {
		?>
		<div id="theme-installer" class="wp-full-overlay expanded">
			<div class="wp-full-overlay-sidebar">
				<div class="wp-full-overlay-header">
					<a href="#" class="close-full-overlay button"><?php _e( 'Close' ); ?></a>
					<span class="theme-install"></span>
				</div>
				<div class="wp-full-overlay-sidebar-content">
					<div class="install-theme-info"></div>
				</div>
				<div class="wp-full-overlay-footer">
					<button type="button" class="collapse-sidebar button" aria-expanded="true" aria-label="<?php esc_attr_e( 'Collapse Sidebar' ); ?>">
						<span class="collapse-sidebar-arrow"></span>
						<span class="collapse-sidebar-label"><?php _e( 'Collapse' ); ?></span>
					</button>
				</div>
			</div>
			<div class="wp-full-overlay-main"></div>
		</div>
		<?php
	}