I'm using Kubuntu 16.04. When I launch gparted from GUI it looks like an "old windows 95" (or something like that) application, but when I type
sudo gparted it has this Kubuntu look.
I saw similar behavior on Xubuntu 16.04 with SimpleScan
Themes are implemented on a per user basis. Installing one for your user won't result in a change to the root theme. launching with sudo results in the use of roots profile rather than your own. You can prove this to yourself rather simply. the command
sudo -i will launch an interactive shell with sudo permissions. None of the scripts in your ~/bin directory will respond to tab completion like they would if you were under your own profile and the command
pwd clearly shows you are in the
/root directory as sudo rather than the
home/you directory (where you is your username).