Could not edit the custom.css

Inexplicably it is not possible to edit in the theme options/styling (where the custom.css field is) something in my custom.css. I have to sites with the same theme, but on one site the field is white instead of black background, where I could edit the custom.css.

the editable page looks like this:
https://www.dropbox.com/s/3afhzgmwfzfsvxy/custom%20css%20editierbar.jpg?dl=0

and the side with the white background like this:

https://www.dropbox.com/s/3vrcrmttqw9jxcw/Custom%20css%20nicht%20zu%20editieren.jpg?dl=0

Does anyone have any ideas how I can edit the custom.css again?

Thanks,
cquadrat

• This topic was modified 4 years, 11 months ago by Jan Dembowski. Reason: Moved to Fixing WordPress, this is not an Everything else WordPress topic