I don't use Mint with KDE (not yet
) but I have another distro with KDE in dual-boot. I suppose that the manipulations are the same although this distro has not Dolphin.
You should find normally the KDE Center of Configuration
. In it click on Peripherals
then on Display
You can also right-click on your desktop and choose Configure the Desktop
I use my distros in French so I hope I have translated correctly the french words
If not I'm sorry
K.I.S.S. ===> "Keep It Simple, Stupid"
"Simplicity is the ultimate sophistication." (Leonardo da Vinci)
"Everything should be made as simple as possible, but no simpler." (Albert Einstein)