Is are support to theme the File/Edit menus?

I’m on Linux and I recently updated to 3.4.12, from an old version 2.11.11. I noticed the File/Edit menu appearance has been changed to have really rounded corners and also seems the each item is much bigger than previously.

I don’t know if the menus were native previously, but I’m using Cinnamon, without any rounded corners, so the menus looks really out of place. The design seems to be in line with the modern Gnome look, but I would guess they are rendered by Electron.

Is there any support to change the appearance of these menus using custom css?

You are correct that this part of Joplin comes from Electron. There is also no way to change its appearance by the user.