[kubuntu] Window configuration

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http://ubuntuforums.org – Hey there. I'm relatively new to KDE, I have been playing around with all the options, but there's one I can't seem to find. How do I change the inside of a window? In the sense, of the rectangular box from under the title-bar to the bottom of said window. I cannot find any option in system settings to edit this, yet I know it can be done. Where should I be looking? Thanking you all! (Hardware)