Converting metacity-theme-3.xml to CSS

I have an old Metacity titlebar theme I would like to convert to a CSS format for GTK3/4. It is my understanding the GNOME deves will eventually break compatibility for the older Titlebar themes, so I’d like to find how to convert the old Metacity files to the newer format, so that I can have proper and separate Titlebars (and not HeaderBars).

This would be to use with the Muffin window manager rather than Mutter, in case it needs different tags.

It is my understanding the GNOME deves will eventually break compatibility for the older Titlebar themes

We did that in 3.16, back in 2015. There was no conversion tool at the time, and I doubt there has been one in the meantime (the old metacity format is very different from CSS).