Hi All,
I've finally found the "(system settings) appearance/gtk styles and fonts" option, you need to install the trinity-gtk-qt-engine and trinity-gtk3-qt-engine packages to get it to appear. After using this to choose the "oxygen-gtk" theme, this has mostly solved my problems. The Mageia control centre is still failing to pick up my alternative theme settings (although the scroll bar "jumpiness" has been fixed"). Other apps like Firefox are now displaying normally.
Thanks for your help!
Tim W