Default syntax highlighting for dark theme

Summary

Beta dark theme doesn't have default dark syntax highlighting. It's not a big deal if you use it yourself. But if you set dark theme as a default theme, all code in public repos for guests will be with white background.

Steps to reproduce

Set dark theme as a default theme, open any public code as guest.

What is the current bug behavior?

If you set dark theme as a default theme, default syntax highlighting still be white.

What is the expected correct behavior?

If you set dark theme as a default theme, default syntax highlighting should be dark. OR We should have an option, f.e in gitlab.rb for changing default syntax highlighting, like we have it for theme.

Relevant logs and/or screenshots