R1:b7aacaa4d30b

Differentiate Remarkup header sizes more clearly

Summary:
Ref PHI1275. Previously, see T591. See also T7963. Headers are currently very visually similar to one another, and similar to the text size:

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I think the design intent was to make it hard to make bad-looking documents, but all the headers end up being very samey.

Differentiate the sizes of the headers better so they're much more obvious (e.g., when scrolling through a document) and the different levels are more distinct.

This might be…