Extend Site New Use case

extend-site-new.php

<?php

add_action( 'network_after_site_new_form', function() {
	$themes = wp_get_themes();
	?>
	<table class="form-table">
		<tr class="form-field">
			<th scope="row"><label for="my-plugin-theme">Select your theme</label></th>
			<td>
				<select name="my-plugin-theme" id="my-plugin-theme">
					<?php foreach ( $themes as $theme ): ?>
						<option value="<?php echo esc_attr( $theme->get_stylesheet() ); ?>"><?php echo $theme->title; ?></option>
					<?php endforeach; ?>
				</select>
			</td>
		</tr>
	</table>
	<?php
});

add_action( 'wpmu_new_blog', function( $blog_id ) {
	if ( ! isset( $_POST['my-plugin-theme'] ) )
		return;

	switch_to_blog($blog_id);
	switch_theme( $_POST['my-plugin-theme'] );
	restore_current_blog();
});