fix(Storybook): fixes white flashes between loading

[arwehrman]

Description

Prior to 6.5, we were able to set appBG in the custom Storybook Theme to set the background in the Preview iframe. This is no longer the case. There are possible fixes coming but have not seen them actually committed to a release yet. storybookjs/storybook#24575

The other known issue is a white flash when clicking from a Story to the Docs of that Story. This is related to some classes being displayed and not displayed during 'loading' of the pages. The CSS overrides added in this PR are to fix those issues until Storybook fixes them. See this issue

This PR makes a few updates to the Storybook styles configuration:

• Adds CCS style overrides for style overrides the known appBG and the loading issue styles when clicking between Stories and Docs

References

LUI-1200

Testing

• clone pr
• go to multiple Components
• click from the Story to the Docs and back. You should no longer see the white flash

Automation

Checklist

• all commented code has been removed
• any new console issues have been resolved
• code linter and formatter has been run
• test coverage meets repo requirements
• PR name matches the expected semantic-commit syntax

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