In terms of resources, there are a few that I have found.
There is also a TextMate package , although it is several years old.
Developing for Tumblr is a bit of a pain since I do this by setting up a test tumblr to use, re-hack or post each type of message I want (photo, photoset, audio, text, etc.). I work on HTML locally and customize it the way I want until I find out that I can do almost everything I need to achieve with CSS only. Then I place any resources on my server (CSS / JS / etc.), I use the theme editor on a test topic to update the HTML code, and then everything I need to do can only be done in my remote assets. If I need to edit the HTML, I will make it locally and then bring it back to the theme editor.
This is not the nicest way to work, but I have done about 4 in this way, and it works well for me.
Jonmack
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