As of 10 Sep 2025 this has been fixed. you can now activate editing css in settings-> general. See the CSS page for more details.

In the most recent update to Glow blogs we updated the version of Jetpack that we use. Jetpack no longer supports Additional CSS. This is no longer available from the customiser or in the Styles of the Site Editor.

CSS already added to sites continues to function. Native WordPress support Additional CSS but not in multi-sites like Glow Blogs. The developers are investigating how to implement this and we hope to enable Additional CSS soon.