There two ways to proceed:
If your installation of Kubuntu is not corrupted then logout or reboot and select Plasma session on login screen.
If Kubuntu installation is corrupted, then (re)install Kubuntu desktop back by
sudo apt-get update
sudo apt-get install --reinstall kubuntu-desktop^
While executing above command confirm using SDDM as login screen, ...
I had the same problem on Zorin OS. Neither of these helped. I have the G613 Keyboard and I've tested the keys. Even though some of them are not recognized, Alt and Shift are.
What helped me was to check the Switching to another layout Alt+Shift checkbox in the Gnome Tweaks tool under the Additional Layout Options submenu of the Keyboard & Mouse menu.
Here's a low-effort answer, if anyone is trying to simply switch between display managers.
Install xdm (we'll be removing it later)
sudo apt-get install xdm
It'll then show an option to switch your default display manager, you can select the one you want from there, and it'd (hopefully) work.
Uninstalling xdm isn't necessary but you may do it using
sudo apt ...
If you need something super quick and simple, try
which gives you the wall clock time from when you run the command to when you press the ENTER key again. (The read command reads a line of input from STDIN.)
I know the OP is on 16.04 but it has been 3 years since this question was asked and now I am answering for 18.04 +
On Ubuntu 18.04 +
You can check gnome-tweaks first of all before going with other stuff
sudo apt install gnome-tweaks
Then open gnome tweaks
Then go to appearance > Then application theme > Yaru > Icon theme Yaru ...
the last 5 years I worked with Ubuntu 16.04 and I really enjoyed it.
Now after it's LTS is over I installed a new Ubuntu 20.04 together with unity desktop.
But then I also faced this missing feature of highlighted workspaces in the quick switcher using Keyboard shortcuts [Ctrl + Alt + ArrowKeys].
If there are open windows, in the preview they are brighter ...
You can do this with the GNOME extension Auto Move Windows
After installation you can configure how the extension works with your applications.
For it to work effectively you can select which applications to start by selecting
Startup Applications Preferences.
I found that this works on Ubuntu 18:
# Configure dconf to disable screen lock
sudo mkdir -p /etc/dconf/profile/
sudo tee /etc/dconf/profile/user <<EOF
sudo mkdir -p /etc/dconf/db/local.d/
sudo tee /etc/dconf/db/local.d/10disable-lock <<EOF
sudo dconf update