Source

File: wp-admin/includes/upgrade.php

function make_site_theme_from_oldschool( $theme_name, $template ) {
	$home_path = get_home_path();
	$site_dir  = WP_CONTENT_DIR . "/themes/$template";

	if ( ! file_exists( "$home_path/index.php" ) ) {
		return false;
	}

	/*
	 * Copy files from the old locations to the site theme.
	 * TODO: This does not copy arbitrary include dependencies. Only the standard WP files are copied.
	 */
	$files = array(
		'index.php'             => 'index.php',
		'wp-layout.css'         => 'style.css',
		'wp-comments.php'       => 'comments.php',
		'wp-comments-popup.php' => 'comments-popup.php',
	);

	foreach ( $files as $oldfile => $newfile ) {
		if ( 'index.php' === $oldfile ) {
			$oldpath = $home_path;
		} else {
			$oldpath = ABSPATH;
		}

		// Check to make sure it's not a new index.
		if ( 'index.php' === $oldfile ) {
			$index = implode( '', file( "$oldpath/$oldfile" ) );
			if ( strpos( $index, 'WP_USE_THEMES' ) !== false ) {
				if ( ! copy( WP_CONTENT_DIR . '/themes/' . WP_DEFAULT_THEME . '/index.php', "$site_dir/$newfile" ) ) {
					return false;
				}

				// Don't copy anything.
				continue;
			}
		}

		if ( ! copy( "$oldpath/$oldfile", "$site_dir/$newfile" ) ) {
			return false;
		}

		chmod( "$site_dir/$newfile", 0777 );

		// Update the blog header include in each file.
		$lines = explode( "\n", implode( '', file( "$site_dir/$newfile" ) ) );
		if ( $lines ) {
			$f = fopen( "$site_dir/$newfile", 'w' );

			foreach ( $lines as $line ) {
				if ( preg_match( '/require.*wp-blog-header/', $line ) ) {
					$line = '//' . $line;
				}

				// Update stylesheet references.
				$line = str_replace( "<?php echo __get_option('siteurl'); ?>/wp-layout.css", "<?php bloginfo('stylesheet_url'); ?>", $line );

				// Update comments template inclusion.
				$line = str_replace( "<?php include(ABSPATH . 'wp-comments.php'); ?>", '<?php comments_template(); ?>', $line );

				fwrite( $f, "{$line}\n" );
			}
			fclose( $f );
		}
	}

	// Add a theme header.
	$header = "/*\nTheme Name: $theme_name\nTheme URI: " . __get_option( 'siteurl' ) . "\nDescription: A theme automatically created by the update.\nVersion: 1.0\nAuthor: Moi\n*/\n";

	$stylelines = file_get_contents( "$site_dir/style.css" );
	if ( $stylelines ) {
		$f = fopen( "$site_dir/style.css", 'w' );

		fwrite( $f, $header );
		fwrite( $f, $stylelines );
		fclose( $f );
	}

	return true;
}