How to change the diagram style color, etc.?
 

I'm using OmniGraffle Pro to build a fishbone type decomposition using the included "Lines" diagram style. I want to change the green of the third level to be more of a forest green. I went in and changed the color for each line and text element, but then when I add to the outline, it reverts back to the lime green for new outline elements. How do I change the style so that when I add stuff it will follow my new style choices rather than what's programmed into it for the "Lines" diagram style?

You want to make a new diagram style. File->New Resource->New Diagram Style..., select the Lines diagram style. Make your changes. File->Save As... your new diagram style (it will default to the Diagram Styles directory so it shows up in the Diagram Style chooser). Now you should be able to apply that new diagram style to your existing document with Format->Choose Diagram Style...

See "Using diagram styles" in the built-in help function for more details.

It's worth mentioning that you can take an existing document, make all of the layout and style changes that you want, and then export it as a Diagram Style via the File Menu -> Export… instead of starting from scratch.

Hope that helps,