I guess that the theme doesn't provide colors for the scopes used by git diff. Position the cursor on something you'd like to see in some color (such as a line added, say). Then open the console and enter this:
I did that with the cursor on the word "class" in a Python file and got this:
u'source.python meta.class.python storage.type.class.python '
So if my theme were to specify a color for "storage.type.class.python", say, then I would be happy.
I don't use git myself, so I haven't looked at the result of git diff.