Hi all, I’m really sorry for the lack of replies! I haven’t been using Tumblr for awhile, so I will no longer be answering questions due to weeks of being inactive ~I’m sure you’ve all moved on from the question now. But I’ve been getting loads of requests on how to create a Tumblr theme. Unfortunately, I am very bad at explaining so I’ve provided you with a simple base stylesheet (two collumns) which you can get here.
You can read more about how to edit/customize them, here. If you’re having trouble, ettudis has a great tutorial page on it! Just play around with it, add fonts (google.com/webfonts), colors and your own style to it :)