fix: set xblock_iframe background to transparent [FC-0062]

[rpenido]

Description

This PR sets the background of the xblock_iframe wrapper to transparent, to fix the component preview in the course authoring mfe.

Before:

After:

Testing instructions

• Checkout this branch
• Open the Library Authoring Page and check the background of the component in the sidebar.

---

Private-ref: FAL-3859

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[pomegranited]

@rpenido This is no longer happening on the tagging-preview sandbox, but I don't know why.. (I checked and this change hasn't been deployed there :) )

And since it's not happening in my tutor dev stack either, I don't think we should merge this change.

[bradenmacdonald]

Weird - this was still happening for me, and I wasn't aware of this PR, so I made a similar fix in #35598

[rpenido]

Great @bradenmacdonald!
I'm closing this in favor of #35598.