CarPlay always in dark mode ?

No, I'm not having the problem. Iit may be an issue with how your car determines night mode.

My car is the same way. If I set headlights off, from "auto" which they typically are, then the screen goes to light mode.

I had the same issue. Turns out, that in my case the car headlights must be set to auto to work. (A bit stupid that it’s not possible to use the time of day or the phones mode)

My car is a Peugeot 208

Go to settings, then “Privacy”, then “Location Services”, scroll all the way down and go to System Services. Make sure to turn on “System Customization” and “Setting Time Zone”. Apparently this allows the interface to switch between dark and light based on location.