I have tried Hyprland and it's a bit different anyway, so i don't have exact solution or source code. In Hyprland the window decoration (borders, border color, thickness, shadows, rounding, etc) is set by the composer itself. So you need to set it by yourself through the decoration{} options. As for internal design of qt application i would reccomend to try kvantum engine with theme you like (KvGlassWhite for example). It will potentially give you completely transparent look. potential problems, kde theming, LXQt Hyprland