fix: set default theme according to preferred color scheme

[fabioatcorreia]

Issue:
Currently, the theming documentation is not completely correct because "Unless you've set your preferred color scheme as dark" is not working.
ref #6088

What I did

Updated the theme under the defaultState to reuse the create function, which, when called without arguments, already returns the ThemeVars according to the preferred color scheme.

How to test

Open up a browser which supports the preferred color scheme preference, with any Storybook without a custom theme and, check that its theme is according to your preference.

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[ndelangen]

Hey @fabioatcorreia that's really sweet! Thank you for the contribution!!

[ndelangen]

@shilman perhaps though not technically a BREAKING change, it might be perceived as disruptive one for users...
After upgrading their storybook might be dark all of a sudden, when they did not expect this.

WDYT, do we release this in a minor, or wait for a major?

[ndelangen]

We currently support this:

addons.setConfig({
  theme: themes.dark,
});

Users can currently do this:

addons.setConfig({
  theme: create(),
});

It would be great if users could configure:

addons.setConfig({
  theme: 'auto',
});

This would avoid the disruptive change but also allow users to easily opt-in to the automatic theme switching.

[fabioatcorreia]

Sorry, but I couldn't understand if that last comment was meant for me or @shilman. @ndelangen?

[ndelangen]

@fabioatcorreia I was expressing concern that users of storybook might find the change disruptive (storybook could turn to dark-mode by default) for a minor version bump.

And I was mentioning that users could already opt-in to the behaviour this PR sets.
I'm not sure how to proceed. I agree this PR should be merged, I just don't know when.

[literalpie]

I think this is a great feature and I would love to have it, but I think adding dark mode could be breaking if any extensions are installed that do not support dark mode, or if someone is customizing the styles of SB.
If this does get merged, I would suggest clearly documenting a way to turn it off for anyone who has issues.