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I finally went direct to support, and I learned that this is not possible at this time.
Per Marketo Support:
At this time for emails they cannot be beyond what is available. This is, however, on the roadmap for a feature to be added. They are downloaded as images that are generated on server so even CSS cannot be used to modify them at this time. I do know the product manager is aware and looking at adding it, but there's not any timeline for it, I spoke to him personally a while back when I had another customer ask about this.
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