A well done blog is one where its easy to read above anything else. The layout of a website is a reflection of the designer and the content producer. Getting your message across means thinking about the person who will be reading it first. A busy background, non complimentary colors and too much eye candy can get in the way of your blog being its best.
There are a lot of themes on Tumblr choose the one that suits your personality. Your theme should help you communicate what you want to say in the best possible way.
The Tumblr Years of Tony Burgess 