rrt style: Missing syntax highlighting for .diff files

Greetings,

I am using the theme rrt and I love it! But I've noticed that it does not syntax highlight .diff files at all.

Take this sample:

- foo
+ bar

It works perfectly with the monokai style, while rrt does not provide any highlighting. However other styles seem to be affected, too.

Thanks in advance, I hope it can be resolved. I did not seem to have any issues with rrt with other file formats and I do love the theme! :)

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