
If I go to System Settings > Style and select (for example) QTCurve style and click Apply, it appears to work. The System Settings takes on the new setting and running apps (for example, Akregator) will take on the new setting. But: then open an app (Dolphin for example, or even close and reopen System Settings) and it is still using Oxygen! It never keeps the new setting. If you go back to System Settings > Style, it shows the style you selected, even though everything is ignoring it and still using Oxygen. Logging out and back in makes no difference.
In case it matters, some details: using NVIDIA drivers (DDM recommended, installed with DDM); desktop effects settings are: Composting type: OpenGL 1.2 (videos can be choppy if I use any other type) and Raster. I have installed oxygen-transparent and QTCurve installed from the repos.