Have created Tumblr theme named Sandwitch. I was going to make tumblr-blog look like showdown blog-engine style I’ve made, and seems the result in the end is even better!
You can turn on tweets translation, allow Disqus comments and… write posts with source code highlighting!
To use source code highlighting it is required to make first line in your code blocks to be something like #!xml, so I recommend to write them using Markdown-edtior (it can be selected in your profile preferences). SHJS (copied to static.tumblr) is used to make it work. You also must specify programming languages used in blog manually (not just by checking “Highlight code” checkbox) in the sort of: ['html','css','xml','javascript','java','python','sh']. The languages you can enable are: html (HTML), xml (XML), css (CSS), javascript (JavaScript), python (Python), java (Java), ruby (Ruby), sql (SQL), sh (Unix Shell), php (PHP), cpp (C++), csharp (C#).
To translate tweets, set your “Twitter username” at theme customization.
To turn on Disqus, set your “Disqus shortname” at the very same settings.