In the Grasshopper menu, go to File→Special Folders→Settings Folder. Then close down all instances of Rhino and delete the grasshopper_gui.xml file. This will revert all UI colours back to default settings.
In the Grasshopper menu, go to File→Special Folders→Settings Folder. Then close down all instances of Rhino and delete the grasshopper_gui.xml file. This will revert all UI colours back to default settings.