Add more styles for source view

[IdrissDimson]

It uses slightly better syntax highlighting than the currently implemented one. I also find it easier on the eyes.
https://craig.is/making/rainbows/

[pierremtb]

@Viruska It is not my taste, but that's only my opinion ;). However, it might be complicated and would surcharge the already surcharged source page to use syntax highlighting from another provider. Maybe you could pick out some styles you like there : https://highlightjs.org/static/demo/ (A lot are available). So I can implement them.

EDIT : Oh I though we were on Liri Browser repository. But.. which source view are you talking about ?