"su " and "su -" do things rather differently
"su -" moves you to the ROOT home folder "/root "and uses the root account $PATH
"su" used the normal users $PATH
a gui program will not normally launch if you use "su"
but will launch if using "su -"
you might WANT to set the root desktop theme to something you HATE , like say "hi-contrast "
that way you KNOW you are using dolphin AS ROOT and know at a glance the DANGERS