The tweaks-menu is messed up in the last version, you can edit the .ini-file directly instead; after editing the .ini power off and reboot and go to library again to activate the new settings...
A new tweaks-menu is in the making by ah-
I for now use the previous to last version because I have not started to use shelves anyway and can wait for the feature becoming more solid or included by Kobo in one of the next firmwares...
