Use default theme when ThemeProvider isn't included in the tree

[mlynam]

Describe the bug
When using testing a component that uses useThemeUI the them object is initialized to null.

To Reproduce
Steps to reproduce the behavior:

1. Unit test a component that uses the useThemeUI hook.

Expected behavior
Without a ThemeProvider in the tree, the theme object returned from useThemeUI should return a default that includes expected properties so we don't need to use a ThemeProvider for every test.

[flo-sch]

Wouldn't that be anti-pattern with how theme-ui works?

Not sure which test runner you are using, but with jest, you can easily add a "wrapper" to provide a theme for your unit tests (ideally the same theme that you use in your app)

It is even illustrated the official documentation from testing-library (if that is the one you are using):
https://testing-library.com/docs/react-testing-library/setup

Most of the test runners and utilities out there allow for wrapping components ;)

[jxnblk]

I'm guessing this is related to the notion of a "default theme", which might be removed in v1: related #832

[lachlanjc]

I think the direction we’re moving in (as Brent mentioned) is away from having a default theme & this isn’t something we’re planning to support.

@hasparus close?

[mlynam]

Understood.