Nit: change the color of the warning banner

[ambv]

Now that the project is in beta (congratulations!), we no longer need BIG SCARY WARNINGS. Instead, pastel yellow would be clear enough without being a distraction anymore. It would also compose with the blue of the theme to form the Pythonic colors 😄

I realize this is a nit but I hope since this is trivial enough to implement, it's not too much to ask.

[brainwane]

@nlhkabu I defer to your thoughts here!

Thank you @ambv for your suggestion.

[brainwane]

Thanks again @ambv!