I created the Child theme for Sydney. there are 2 files and style.css and functions.php.
If I activate it the site loses the theme settings as header image that are present in the site. which files I need to put in order to have a working child theme.
Check out our pre-made Sydney child theme for comparison. Theoretically child theme is a separate theme which is identified by theme’s folder name. Thus you’ve to sync the settings you’ve done when main theme was being active. It can be done by doing the steps as explained in this documentation. Adjust the folder name to exactly match with your child theme.
this is a explanation how change to Sydney pro.
so I do a backup of sydney I active a child thema and I put a backup. it is ok. which benefits I have to have a child?
Yes, you’re right. That’s aimed for Sydney Pro. However it works the same way if you want to transfer the settings have applied when main theme is active. Check out the plugin page that’ll explain you some more details of how it works.
Child theme system is provided by WordPress to allow users to extends the theme’s design and functionalities. As theme version often be updated, editing main theme isn’t an ideal way to do. Thus child theme becomes a perfect solution. You can read the details about child theme on codex: https://codex.wordpress.org/Child_Themes.