Look into improving contrast for the code view

[bobsilverberg]

This is based on feedback from the meeting we had today to discuss the code manager.

The contrast when viewing code is considered less ideal than it is in the old tool. I notice that we had an issue earlier (https://github.com/mozilla/addons-code-manager/issues/774) which addressed contrast for code, but only in text files. It's probably worth referring back to that issue to see what was done, but I believe this new issue is about improving contrast for all files displayed in the code viewer, not just text files.

It was also mentioned that the font was preferred in the old viewer, but I'm not sure if that's the same issue about contrast, or if it's about the actual font used. It was mentioned that perhaps reviewers just need time to get used to the new font.

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