As I read it, you need to change the GUI Skins used by your GUI boxes.
The default GUI skin that's used is defined in Menus > Menu Settings in the Default GUI Skin Settings, the default can be overridden in each GUI box individually.
ORK uses mostly the Box, Button and the various Vertical Scrollbar parts of a GUI skin.
E.g. to change the background of your GUI box, that's handled by the Box style.