[NFR] Support dark mode

Now that MacOS Mojave dropped and android and windows already supporting it would it be possible to allow some changes to the style when we are in dark mode? Apple has a media query @media (prefers-dark-interface) for detecting that we are in dark mode not sure about the rest. I found a good read while searching for this. https://litmus.com/blog/email-designers-next-challenge-apple-dark-mode

** Looks like @media (prefers-dark-interface) is still a private api. Maybe we could leave this open until there is a public interface?

[ruudboon]

Was thinking about something like

<mj-style interface="dark">
</mj-style>

But maybe the designers should just fix this with a normal media query when it’s available. Feel free to close.

[notflip]

You can override any attribute with css, mj-attributes in this case are just applied as inline style.

Ah I see, so using !important to overwrite the inline style, you mean?