Create backgrounds for unified theme

[evwilkin]

Create backgrounds for unified theme.

Glass transparency of components has the potential to negatively impact a11y, especially if the background does not provide enough contrast. It is also possible that legibility of text and icons is degraded as well if the contrast is poor.

Appropriate backgrounds for both light and dark mode are needed for both user testing and a11y testing to ensure positive outcomes for both.

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Jira Issue: PF-3501