make lean's logo visible when using dark mode

Stream: general

[Bulhwi Cha]

It's hard to see Lean's logo when using dark mode on a web browser or GitHub. A non-transparent image will solve the problem.

lean-firefox.png
lean-github.png

[Eric Wieser]

For the former, it's possible to pick between dark and light favicons based on the system dark/light theme, I believe

[Anne Baanen]

We could also apply a thin white outline to the letters, that should work universally and IMO look better than putting everything on a white background :)