/*
Theme Name: mytheme
Description: this is my theme.
Author: admin@admin.com
Version: 1.0
Template: oldtheme (theme Folder Name you want to use for parent theme)
*/
Use this code in style.css file of child theme file
/*
Theme Name: mytheme
Description: this is my theme.
Author: admin@admin.com
Version: 1.0
Template: oldtheme (theme Folder Name you want to use for parent theme)
*/
Use this code in style.css file of child theme file