The button to open the additional window configuration on the left bottom of a theme (Breeze, Oxygen, ...) in "Window Decoration" in Systemsettings is from my point of view placed like it belongs only to the preview of the theme. 
For me it is therefore not easy to spot that it offers additional functionality. Besides this it is redundant, as the same button is shown for every theme again even if not active. Therefore I would like to suggest to move it as a button on the right bottom like the border size or follow another clear design guideline.