Diff syntax highlighting in Github Markdown

I’m writing documents that should explain code in C# using Markdown.

I use the ```csharp to get csharp highlighting.

I sometimes want to highlight something specific in the code using bold or anything.

I know about <pre> etc… but it takes away my csharp highlighting.

Best case scenario – some way to highlight code in the ```csharp section.

Next best thing – I can write the code as diff – using + and – to highlight stuff, but how do I tell Github to highlight diff syntax with the red and green backcolor?

Is there a way to use both diff and csharp syntax highlighting?