using child theme on multisite

I’m reading about setting up a child theme and have already made a folder for it in the file manager of the cpanel. I haven’t created the css file yet.

I’m new to WP Multisite and working with child themes. I probably should have created the child theme prior to overhauling the main site, but I didn’t. I now have the main site pretty much like I want it, making changes to the parent theme defaults (colors, header, background, menu…) and adding plugin-ins and content. I’m almost ready to start adding the subdomains, which I want to look a lot like the main site. Probably only changing up the menu and one of the sidebars, and making the header photo specific to the subdomain. The header image will basically have the name of the chapter location added to the main pic.

Will all of the changes I have made transfer to the child theme or will I have to start over?