UI: Default white preview background

[JReinhold]

Closes #21080

What I did

Set the preview iframe to default to a white background. Previously it was transparent, meaning that Storybook in dark mode would cause a dark background, which is unexpected given that browsers default to a white background - only the user's code should change the background color, not Storybook's code.

Setting the color on the iframe element ensures that user code that sets it on html, body, or something else within the iframe still overrides it.

I've tested that

1. this still works with the backgrounds addon
2. it's still possible for user code to change the background, even based on browser theme, with:

@media screen and (prefers-color-scheme: dark) {
  body {
    background-color: darkblue;
    color: lightgray;
  }
}

How to test

1. Open one of the published sandboxes (eg. this one:)
2. Navigate to one of the page stories:
3. Change your browser color scheme to dark (see screenshot) and reload
4. see that Storybook UI goes dark, but the preview stays white.

Feel free to test this out locally with custom css in stories.

Checklist

• Make sure your changes are tested (stories and/or unit, integration, or end-to-end tests)
• Make sure to add/update documentation regarding your changes
• If you are deprecating/removing a feature, make sure to update
  MIGRATION.MD

Maintainers

• If this PR should be tested against many or all sandboxes,
  make sure to add the ci:merged or ci:daily GH label to it.
• Make sure this PR contains one of the labels below.

["cleanup", "BREAKING CHANGE", "feature request", "bug", "documentation", "maintenance", "dependencies", "other"]

[shilman]

Can you please add a note to MIGRATION.md?

[JReinhold]

I felt I heard someone saying that SB dark mode (in manager UI) was only introduced in 7.0, which is what's caused this bug. But my memory might be wrong?

[MichaelArestad]

I am noticing on cra/default, the light and dark themes don't seem to do anything, but I don't think that's a regression with this PR. The backgrounds addon still works just fine. Nice work!

[MichaelArestad]

Can you also change the Backgrounds addon to not default to light mode? (in the "cleared" state)

[JReinhold]

Can you also change the Backgrounds addon to not default to light mode? (in the "cleared" state)

I'm not sure what you mean by this @MichaelArestad . When it's cleared it doesn't set anything, as if it wasn't there?

[MichaelArestad]

@JReinhold
@yannbf mentioned that Before you made this fix, the Backgrounds addon (used on the Example storybook) was changed to default to light so that the background color wouldn't change when dark mode was enabled. If that's the case, we should revert that change.

[JReinhold]

Aha I see what you mean, are you referring to #20982? I've reverted it as I think it makes sense. Any objections @ndelangen?

[ndelangen]

None @JReinhold