I was looking for the same, but It seems that You can´t sort the bookmarks, at least I dind´t find It or How to do that.
I was happy that the bookmarks returned, but they forget to implement that basic feature, sort Your Bookmarks by name! Now I have maybe 100 bookmarks sort randomly...
I found It!
You can actually sort the folders and their content, just go to Bookmark Menu, in the top left Opera Icon, It will show You the option to Sort alphabetically when You right click the Folder that You want to sort.