Test this:
Create a folder “mytheme”.
Create a file “style.css” inside mytheme folder.
Add this text inside style.css:
/*
Theme Name: Twenty Twelve Child
Theme URI: https://example.com/
Description: Child Theme
Author: John Doe
Author URI: https://example.com
Template: twentytwelve
Version: 1.0.0
*/
@import url("../twentytwelve/style.css");
/* =Theme customization starts here
----------------------------------------------------- */
p { font-size: 50px;}
Now zip the folder “mytheme” -> name it mytheme.zip (or something else)
Upload and activate – Open your site and check if it works.
If it works, then everything is alright.
**You just have to add only those files from parent theme (original theme) – which you want to make changes.