[Feature Request] Add black theme

[breathiness]

Hi, I really like this project.
But currently only the light theme is available. It doesn't really match the GitHub page with the black theme.
I hope you can add black theme.

[ethomson]

Hi, thanks! I don't think that I can do that, though. (At least not in a way that you could embed it.)

GitHub's just asking for an image and (I don't think) giving any information about whether you're in light or dark mode.

And (I don't think) there's a way in markdown to mode select.

I'd love to be wrong about that! It's tough to embed images in markdown without knowing whether you're in light or dark mode, even transparent pngs suffer.

[breathiness]

Actually what I want is not to switch automatically, but to be able to choose the theme when adding. Like this project: https://github.com/DenverCoder1/github-readme-streak-stats

[ethomson]

Aha, that seems do-able!