Add a dark mode version of logo

[Saransh-cpp]

• dark mode version of logo
• no sidebar title
• dynamic rendering of dark/light logo in the README file

See FluxML/Zygote.jl#1306

The white Flux logo looks a bit weird to me, but maybe because I am used to seeing the black Flux logo?

PR Checklist

• Tests are added
• Entry in NEWS.md
• Documentation, if applicable

[darsnack]

Maybe a white outline on the arrow is better than a totally white arrow?

[ToucheSir]

Maybe a white outline on the arrow is better than a totally white arrow?

I concur. At the very least, it would be nice to have distinct shades for the arrow and outline.

[mcabbott]

Shall we try this out? Seems like a step forwards.

I'm not too sure how @Saransh-cpp made this, but I guess replacing a colour is easier than adding new borders of a different colour.

[Saransh-cpp]

I guess replacing a colour is easier than adding new borders of a different colour.

Yes! It took me some time (not a graphic designer 😬), but here are the designs (see them in GitHub's dark mode) -

(I don't have a very strong preference in this matter. Which one should go in?)

[Saransh-cpp]

Note: The arrow in current Flux logo has a greyish outline.

[mcabbott]

I actually think this looks fine. Maybe there's a design where the text & arrow have boundaries like the "layers" do, but this sounds like a lot of work, and not immediately obvious it would be better. I think you'd still want the bulk of the text white (in dark mode) to match the other text.

Before this PR:

After:

[Saransh-cpp]

conda-forge does something similar with their logo, but the current one looks good to me as well -

[Saransh-cpp]

Also, now that I look at it, maybe we should always use the old logo in README? Flux uses the old logo everywhere (for example - FluxML's GitHub logo/display picture), and most of them cannot be made dynamic.