Different users can have different settings

The root /1/ has own settings. You could change the root's settings by starting the System Settings with the command:

kdesudo systemsettings

Root's gtk apps

As default the root's folder doesn't have the link to the oxygen-gtk.

To fix this you need to copy the hidden file .gtkrc-2.0-kde4 (note the dot !) from your home directory to the /root/ with name .gtkrc-2.0 (note the dot !) /2/, /3/.

Command line:

sudo cp .gtkrc-2.0-kde4 /root/.gtkrc-2.0

If you have the Root Actions Servicemenu /4/ installed you could use it to copy and rename.

System's default mouse theme (KDM cursor)

The Debian and the Debian based are using the alternatives /5/ to set the environmental variables.

:~$ man update-alternatives /6/
NAME
       update-alternatives - maintain symbolic links determining default commands

SYNOPSIS
       update-alternatives [options] command

DESCRIPTION
       update-alternatives  creates,  removes,  maintains  and  displays   
       information about the symbolic links comprising the Debian alternatives       
       system.

       It is possible for several programs fulfilling the same or similar functions 
       to be installed on a single system at the same  time.

To set the cursor theme with the alternatives command line:

sudo update-alternatives --config x-cursor-theme

With KDE gui:

Package: kalternatives /7/ - graphical alternatives system configuration tool

More: Change system default X11 cursor theme /8/.

Sometimes

Quote: "Sometimes, in KDE applications under my ordinary user, fonts spontaneously change to Ubuntu font - why this happens and how can I avoid it?"

Yes, I have noticed this. Why - beats me. The Kubuntu is resetting to the default Kubuntu settings (/usr/share/kubuntu-default-settings/kde4-profile/default/share/config/)

To fix this i have edited the Kubuntu defaults: /usr/share/kubuntu-default-settings/kde4-profile/default/share/config/kdeglobals.

Default Kubuntu kdeglobals has:

[General]
BrowserApplication[$e]=!rekonq
XftHintStyle=hintmedium
font=Ubuntu,9,-1,5,50,0,0,0,0,0
menuFont=Ubuntu,9,-1,5,50,0,0,0,0,0
desktopFont=Ubuntu,9,-1,5,50,0,0,0,0,0
toolBarFont=Ubuntu,8,-1,5,50,0,0,0,0,0
smallestReadableFont=Ubuntu,8,-1,5,50,0,0,0,0,0
taskbarFont=Ubuntu,9,-1,5,50,0,0,0,0,0

Looking my own kdeglobals: ~/.kde/share/config/kdeglobals and copying them to the defaults. Here:

[General]
BrowserApplication[$e]=!rekonq
XftHintStyle=hintmedium
font=Sans Serif,9,-1,5,50,0,0,0,0,0
menuFont=Sans Serif,9,-1,5,50,0,0,0,0,0
desktopFont=Sans Serif,9,-1,5,50,0,0,0,0,0
toolBarFont=Sans Serif,8,-1,5,50,0,0,0,0,0
smallestReadableFont=Sans Serif,8,-1,5,50,0,0,0,0,0
taskbarFont=Sans Serif,9,-1,5,50,0,0,0,0,0

Links:

1. https://help.ubuntu.com/community/RootSudo
2. http://forum.kde.org/viewtopic.php?f=63&t=62352
3. http://kubuntuforums.net/forums/index.php?topic=3092288.msg201654#msg201654
4. http://kde-apps.org/content/show.php/Root+Actions+Servicemenu?content=48411
5. http://www.debian-administration.org/articles/91
6. http://en.wikipedia.org/wiki/Man_page
7. http://packages.ubuntu.com/oneiric/kalternatives
8. http://kubuntuforums.net/forums/index.php?topic=3112235.0

Remove those as well - kubuntu-full is a metapackage, installing it will include it's recommends - uninstalling it does not unfortunately. Only way is to remove the packages as you are doing.

More information on metapackages can be found here - https://askubuntu.com/a/2230/2018

Reply to comment:

No - it's not done that. I would install synaptic and then use that to search for the kde things you have installed. It's one of those jobs easier the second time, next time do it with a terminal and copy the output so you know what gets installed.

For instance

sudo apt-get install kde-full 

get's these http://pastebin.com/QrYvxUf1

You might though find it easier and quicker to get the newest 12.04 daily and reinstall, you can use zsync to update your existing iso if you still have that.