We've done some pretty extensive CSS customisation of the HTML templates, but we've always copied and pasted the code from Studio in to a code editor and then copied/pasted back when we were ready. A bit tedious but at least that gave us some syntax highlighting and auto completion. The language used in the email templates is Apache's Velocity engine and you can do some basic if/else queries on some elements of the user profile data, which can be useful. What I'd really love to see is the ability query the API in the email templates rather than just a limited definite subset of available variables.
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