How to synchronize custom CSS, plugins and their data, notes and notebooks order & empty notebooks (if not synchronized already), and settings?
Copy these from Joplin profile from sender to receiver
empty notebooks should be synced by Joplin itself
1 - Should I close Joplin before restoring the data?
2 - Can I synchronize them using a standalone sync app (noting that no other PCs are in sync for Joplin data)?
- better to close the app, yes
- well, you probably can use standalone sync app (such as syncthing), but noone has ever reported how it goes long term. So exercise caution and backup the valuables. If smt goes wrong, please post your experience
- What do you mean by long term? You mean that they stop developing the app?
- Is it dangerous to synchronize other files you didn't mention in the Joplin profile directory?
There might be a lot of edge cases and weird situations with sync. It's hard to catch them all, so I simply recommend to have a backup of those files.
Syncthing is quite stable and well maintainable, so no, I don't think devs will stop the support
Honestly, I don't know. I presume it's going to be alright, but Joplin profile got a lot of items (hundreds of thousands?). Syncing it all by overlapping the content with Joplin native sync engine seems like a bad idea. So, I'd sync only what is necessary