Qt Creator (and Qt dev tools) are using Qt5, yes, but KDE itself (dolphin, ksnapshot, konsole, etc...) are using Qt4.

Qt 5 applications are not able to use the Oxygen style, so they are probably using Plastique, which would explain their appearance.
As for why the double click preference is not being respected - that is probably because no KDE platform plugin is present yet, as KDE is still in the process of porting to Qt 5 (see the frameworks effort for more information here).