Currently, setting &bg at all re-initializes highlights and reloads
the active colorscheme, even if the actual value of &bg has not changed.
With https://github.com/neovim/neovim/pull/26595 this causes a
regression since &bg is set unconditionally based on the value detected
from the terminal.
Instead, only reload the colorscheme if the actual value of &bg has
changed.