Code block syntax highlighting

Created by: elzannewentzel

Joe provided feedback on the way we style code block syntax highlighting on the blog.

https://about.sourcegraph.com/blog/introducing-migrator-service/ the code block colour seems too bright we also use thin font for code (much thinner than the regular text, which kinda makes it even harder to read)

The way it's styled on the docs site is easier to read: https://docs.sourcegraph.com/batch_changes/how-tos/creating_a_batch_change