I don't think it's actually that difficult. All you do is go to Customize > Theme and look for a little option that allows you to edit the document HTML or something like that. After that, it's like coding a regular website but you just have to use some variables.
I personally haven't gotten around to making a theme myself, but I have edited the one I have. Wasn't that hard. I suggest you just pick a simple theme or something, and then see what you can do and change and so on.
Also, I'm sure there are plenty of people on Tumblr that could be willing to teach you how to make your own theme, or you could even ask some people here on CreateBlog that make their own Tumblr themes (Gabrielle and Sandy are the only ones I know. Not sure about Sandy, though.
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