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.