Specify border color of quote when quoted file is not found

[hackerbirds]

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Description

There sometimes is a visual problem where the quote border becomes black on the "Original file not found part", like so:

I can't 100% fully reproduce which is odd, but it does happen sometimes (moreso with light theme?) so I added CSS to explicitly specify the border color on the appropriate element and make sure it can't happen anymore:

Note that both light theme and dark these use #FFFFFF for this border so I didn't feel the need to consider the individual cases

[scottnonnenberg-signal]

We've already got a fix for this coming in the next release - thanks for jumping in, though!