Hi Lavellotron,

> I'm modifying the files located here:
>
> /Applications/MoI\ v4.app/Contents/Resources/ui
>
> Is this the correct location?

Yes, that is the correct location, the main file to edit is moi.css located there, there are a couple of dark versions of moi.css here:
http://moi3d.com/forum/index.php?webtag=MOI&msg=10199.3

The PNG image files for most of the buttons are in an "icons" subdirectory underneath ui.

Did you have MoI open while you were making changes? They won't show up until you close MoI and relaunch it.

- Michael