During the upgrade you were asked to either keep the version of 05_debian_theme you had modified or to install the new version, because you choose to keep the old version you did not receive the change that would have removed the GRUB_PREFIX line as Colin mentioned. In the future one thing to do would be choose "show the differences between the versions" to inspect the changes between the files and make a decision after that.
Here is the relevant bit of the upgrade log: Configuration file `/etc/grub.d/05_debian_theme'^M ==> Modified (by you or by a script) since installation.^M ==> Package distributor has shipped an updated version.^M What would you like to do about it ? Your options are:^M Y or I : install the package maintainer's version^M N or O : keep your currently-installed version^M D : show the differences between the versions^M Z : start a shell to examine the situation^M The default action is to keep your current version.^M *** 05_debian_theme (Y/I/N/O/D/Z) [default=N] ? n^M ** Changed in: ubuntu-release-upgrader (Ubuntu) Status: Confirmed => Invalid -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/1078653 Title: GRUB_PREFIX not set after dist-upgrade To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/ubuntu-release-upgrader/+bug/1078653/+subscriptions -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs