Add Dark Mode Support

[joshmedeski]

Some of us started the conversation at mailhog/MailHog#302

As a Mailhog user, I would like the UI to support dark mode.

Dark mode is not supported by Bootstrap right now, but a third-party Bootstrap theme might be the easiest way to implement this feature.

I'm happy to help, let me know your thoughts, thanks.

[adpe]

Thanks for raising this issue @joshmedeski

I was able to implement a way with the Bootswatch Cyborg theme (needs some modifications…).

More information under #57.

Greets
Adrian