Missing CSS for syntax highlighting

[jouvin]

I found that there is no CSS defined for syntax highlighting (rendering of verbatim sections enclosed by ``` lines). As a first start, the minima syntax highliting CSS can be reused. I don't know if the problem has to be fixed in Materialize framework itself or in the Jekyll Materialize theme. If you think this is better to open the issue in Materialize itself, let me know and I'll do it.