OmniFocus 2.5 Brings Back Themes Link
The Omni Group just released version 2.5 of OmniFocus for the Mac and it brings back custom styling. They even provide a FAQ for creating custom styles using the OmniFocusColors app or a text editor.
The transition from version 1 to 2 was rough for many people. The lack of theming in v2 was a big complaint for a lot of people I know. I might consider OmniFocus again when it reaches parity with version 1. It looks like they are moving through the feature checklist at a pretty good clip this year.