get_theme_update_available

function get_theme_update_available( $theme ) {
	static $themes_update = null;

	if ( ! current_user_can( 'update_themes' ) ) {
		return false;
	}

	if ( ! isset( $themes_update ) ) {
		$themes_update = get_site_transient( 'update_themes' );
	}

	if ( ! ( $theme instanceof WP_Theme ) ) {
		return false;
	}

	$stylesheet = $theme->get_stylesheet();

	$html = '';

	if ( isset( $themes_update->response[ $stylesheet ] ) ) {
		$update      = $themes_update->response[ $stylesheet ];
		$theme_name  = $theme->display( 'Name' );
		$details_url = add_query_arg(
			array(
				'TB_iframe' => 'true',
				'width'     => 1024,
				'height'    => 800,
			),
			$update['url']
		); // Theme browser inside WP? Replace this. Also, theme preview JS will override this on the available list.
		$update_url  = wp_nonce_url( admin_url( 'update.php?action=upgrade-theme&theme=' . urlencode( $stylesheet ) ), 'upgrade-theme_' . $stylesheet );

		if ( ! is_multisite() ) {
			if ( ! current_user_can( 'update_themes' ) ) {
				$html = sprintf(
					/* translators: 1: Theme name, 2: Theme details URL, 3: Additional link attributes, 4: Version number. */
					'<p><strong>' . __( 'There is a new version of %1$s available. <a href="%2$s" %3$s>View version %4$s details</a>.' ) . '</strong></p>',
					$theme_name,
					esc_url( $details_url ),
					sprintf(
						'class="thickbox open-plugin-details-modal" aria-label="%s"',
						/* translators: 1: Theme name, 2: Version number. */
						esc_attr( sprintf( __( 'View %1$s version %2$s details' ), $theme_name, $update['new_version'] ) )
					),
					$update['new_version']
				);
			} elseif ( empty( $update['package'] ) ) {
				$html = sprintf(
					/* translators: 1: Theme name, 2: Theme details URL, 3: Additional link attributes, 4: Version number. */
					'<p><strong>' . __( 'There is a new version of %1$s available. <a href="%2$s" %3$s>View version %4$s details</a>. <em>Automatic update is unavailable for this theme.</em>' ) . '</strong></p>',
					$theme_name,
					esc_url( $details_url ),
					sprintf(
						'class="thickbox open-plugin-details-modal" aria-label="%s"',
						/* translators: 1: Theme name, 2: Version number. */
						esc_attr( sprintf( __( 'View %1$s version %2$s details' ), $theme_name, $update['new_version'] ) )
					),
					$update['new_version']
				);
			} else {
				$html = sprintf(
					/* translators: 1: Theme name, 2: Theme details URL, 3: Additional link attributes, 4: Version number, 5: Update URL, 6: Additional link attributes. */
					'<p><strong>' . __( 'There is a new version of %1$s available. <a href="%2$s" %3$s>View version %4$s details</a> or <a href="%5$s" %6$s>update now</a>.' ) . '</strong></p>',
					$theme_name,
					esc_url( $details_url ),
					sprintf(
						'class="thickbox open-plugin-details-modal" aria-label="%s"',
						/* translators: 1: Theme name, 2: Version number. */
						esc_attr( sprintf( __( 'View %1$s version %2$s details' ), $theme_name, $update['new_version'] ) )
					),
					$update['new_version'],
					$update_url,
					sprintf(
						'aria-label="%s" id="update-theme" data-slug="%s"',
						/* translators: %s: Theme name. */
						esc_attr( sprintf( _x( 'Update %s now', 'theme' ), $theme_name ) ),
						$stylesheet
					)
				);
			}
		}
	}

	return $html;
}