Hello,
We had to change the dark coding to support the more recent OSs that have been created after High Sierra (from using Custom color to System color) to support dynamic dark/light mode in the Mac OSs of Mojave and later, so High Sierra now has its default light color now, sorry. You should be able to change the colors of the background color (perhaps to black) and the text color (to white) in iReal Pro preferences but this is not the dark theme you would have been used to sorry.