[macos] Dark mode

Hi, 

is there a way to instruct `bat` to use a different theme when the OS (macOS in particular) is using a dark theme/mode?

Right now, I use this "hack" which works fine, but I'm wondering if there is a more native approach to this.

```bash
cat() {
    bat --theme=$(defaults read -globalDomain AppleInterfaceStyle &> /dev/null && echo default || echo GitHub) $1
}
```

Thank you!