Add dark-mode toggler for navbar-light

Hi,

please add a dark-mode toggle for the "light navbar". Downstream on doc.o.o, I added this:

@media (prefers-color-scheme: dark) {
      .navbar-light .navbar-toggler-icon {
        background-image: url("data:image/svg+xml,%3csvg xmlns='http://www.w3.org/2000/svg' width='30' height='30' viewBox='0 0 30 30'%3e%3cpath stroke='rgba(255, 255, 255, 0.5)' stroke-linecap='round' stroke-miterlimit='10' stroke-width='2' d='M4 7h22M4 15h22M4 23h22'/%3e%3c/svg%3e");
      }
}

This is currently an issue e.g. on the wiki where you need to tap a pitch-black area to bring up the menu on mobile.