BackgroundImage - full page cover variant with theme support

[kmcfaul]

Follow-up to patternfly/patternfly#8350

Add new variant for full page BackgroundImage.
Add new prop for per-theme background overrides:

light-default
dark-default
light-felt
dark-felt
HC-light-default
HC-dark-default
HC-light-felt
HC-dark-felt
light-glass
dark-glass
light-felt-glass
dark-felt-glass

Could be a follow up - update Compass examples to use BackgroundImage instead of setting background image via style on html.

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Jira Issue: PF-4078

[github-actions]

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