Google will soon let Pixel owners manually choose their Pixel launcher theme as either dark or light.
When Google released the Pixel 2, it enabled an automatic dark theme in the Pixel Launcher and in quick settings. Currently, Google's devices automatically set this dark or light theme based on the darkness of its wallpaper. So, if the wallpaper is very dark, you'll get the dark theme with dark folder icons, dark quick settings, and a dark app drawer. If the wallpaper is more light, you'll get the light theme.
Well, Android Police noticed someone added an item in the dev preview issue tracker asking for the ability to manually choose the theme, and a Google employee has recently marked the item as fixed. The Googler also provided the following response: "We have added support for a Dark theme to be applied to quick settings and launcher... It will be available in a future Android build."
That means, when the update rolls out, which should be when Android P officially releases, you'll simply be able to go into the Pixel's settings menu and then choose the launcher and quick settings' theme. Just go to Settings > Display > Device Theme.