The night theme for the experimental mobile browser is still in its very, very early stages. In fact, you'll only know that it exists when you long press on a link or on an image, and only if Android Pie's Dark Mode setting under Developer Options is set to Always On. Further, even the panels that already come in dark gray still need some work -- some of their text is still in black.
While there's no guarantee that the feature will make it past beta, there's a very good chance that we'll see it at some point. Dark mode is an oft- and long-requested feature for Android, since it's easier on the eyes and could even extend battery life. As 9to5Google noted, we'll likely see more elements, including the browser's other menus, dyed a darker color in the future.