Thanks a lot for the suggestions. Regular expression search would be cool. I was thinking to add a few color themes. But custom CSS to be honest I don't think would work well. In my experience CSS is a little fiddly especially given that the css includes some hacks to get safari on ios working properly which was a big pain.

Auto save is already done, nothing you type is every lost. It's saved immediately (and synced every few seconds).

There is a collapse/expand feature but not for subheadings. If you put --- at the beginning of a line followed by a blank line it'll collapse everything after that, as shown in the demo note.