R1:2bcdaad16cff

Add CSS for Rainbow syntax highlighter

Summary: See D768
Test Plan: Looked at highlighted .rainbow files
Reviewed By: pedram
Reviewers: tristanfisher, jungejason, tuomaspelkonen, aran, codeblock, pedram
Commenters: tristanfisher
CC: aran, pedram, tristanfisher
Differential Revision: 769