We frequently get complaints from macOS users that bat does not work on
their default macOS terminal background, which is white.

Pay the price of slightly increased startup time to get a better default
on macOS. To avoid the slightly increased startup time, simply specify a
theme explicitly via `--theme`, `BAT_THEME`, or `~/.config/bat`.

Note that if there is an error when we check if Dark Mode is enabled, we
behave the same as on Windows and Linux; assume that the terminal
background is dark. This harmonizes behavior across platforms, and makes
bat behave the same as before, when Dark Mode was always assumed to be
enabled.