White background + welcome widget border

[audrasjb]

This is related to https://core.trac.wordpress.org/ticket/62831.

I noticed that the white background was removing the background difference between the welcome panel (the section with a white background).

While it doesn't look shocking to me, this PR also adds a border to the welcome widget.

This is the only issue I found while testing this changeset.
More tests are welcome!

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[audrasjb]

I pushed a follow-up commit to the PR to also use white background on alternative color schemes. Maybe we can choose to keep a grey background on some of them, though.

[audrasjb]

@karmatosed do you have any preference for the welcome box? Black border or no border?
That's a detail we can still update later though.