Currently, in SDDM, the password input and the "go" button are white by default. By changing them to a semi-transparent black, they would fit better to wallpapers. Action buttons like "shutdown", "restart" etc already used a semi-transparent highlight. Note: I couldn't find a proper category for SDDM.
Created attachment 139144 [details] Mojave SDDM theme Mojave SDDM theme make controls white, but semi-transparent. It's another way of making them to fit better with the background.
We're happy with the current appearance. The goal is to have our own unique look, not copy macOS. If you prefer the semi-transparent look, well, that's why SDDM is themable. :)