Default theme

[oliviertassinari]

This issue is part of a series of feedbacks. I been upgrading the Material-UI solution.
Unfortunately, I had to fork the package in order to reach my goal.
Now, it's time to reconciliation.

The issue

It would be awesome if we could be supporting a default theme.
That's something I have been implementing on Material-UI in order to reduce onboarding friction.

[kof]

@oliviertassinari please feel free to add specific code suggestions or even implement it and send a PR!

[oliviertassinari]

Here is an implementation example: https://github.com/callemall/material-ui/blob/926256d70c727b7ea6a668ac40fb3add5faf44cc/src/styles/withTheme.js#L12-L19.