Ok so it seems that the key in org.gnome.settings-daemon.plugins.power idle-dim-time is gone in the new Ubuntu version. Since the old method Increase screen idle dim timeout doesn't work anymore are there any other ways to change this?
It looks like the setting for idle dim time has changed schemas. Use org.gnome.desktop.session idle-delay:
For the future, a great way to find a setting is to pipe list-schemas or list-recursively to grep. That's how I figured this one out:
Hope that helped!