I have also noticed that some KCMs show the fonts differently than others, e.g. inside 'workspace behaviour', the entry 'screen edges' shows everything normally, while in 'general behaviour' everything is a bit blurry. -It seems they don't all follow the same theme, with fonts and checkboxes rendered differently.

Some kcms allow the user to drag their contents vertically (which doesnt make sense if everything fits in the view), while most don't. This also breaks the "move window by dragging anywhere" functionality. The ones i've seen do this are: fonts, user feedback, display configuration, night color, general behavior (inside workspace behavior), notifications, launch feedback, mouse.