I just lost all of my additonal css when I try to create additional GP child themes in multisite environment. It was couple of days of work of trial and error. It was stupid of me not to back the customization up. In MU, I don’t have the editor to add funtions to change the width of the footer widgets. I don’t want the change to another other GP child theme. Oh well, back to more customizations I guess.
The CSS in the Additional CSS field is actually stored with the theme. So if you reactivate the old theme with the code then you should be able recover them and copy and paste into the new theme.
The additional CSS is not stored in the database table? I changed the name of the child theme for the two sites I am testing. I re-activated both but it is not there.
The WP core “Additional CSS” field is stored in the database, but is attached to the theme that is active when the CSS is added.
This is a reason why I still suggest people use my Simple CSS plugin – it’s not attached to any theme, and your custom CSS will remain even if themes are switched.