Updated the code to automatically use a Diff syntax file.
Of course you have to have the proper color definitions within your theme file.
I'm wondering whether the syntax file should be disabled by default or something like that... what do you think?
PS You can get the syntax file here: svn.textmate.org/trunk/Bundles/D ... Diff.plist
Save it as Diff.tmLanguage within your Packages/User folder.