Add theme selector to Storybook

[AlanGreene]

Changes

Update Storybook to allow testing of both light and dark modes for all component previews.

Some components such as the Log / YAML display are designed for dark mode only so these specify an override.

/kind misc

Submitter Checklist

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Release Notes

NONE

[AlanGreene]

/test tekton-dashboard-unit-tests

[briangleeson]

/lgtm

[tekton-robot]

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