Fix diff syntax-highlighting

[ashmaroli]

Since this is subjective, its going to take some inputs from multiple perspective..

Current:

PR Initial Commit:

[ashmaroli]

/cc @jekyll/documentation

[DirtyF]

@jekyllbot: merge +docs

[DirtyF]

We should have kept the bold here but it's already more readable, that's what matters the most. Thanks.

[ashmaroli]

Bold + text-shadow seemed a li'l too much to me..

[ashmaroli]

@DirtyF see the extra borders around the code blocks in the pics above..? I'm guessing it has something to do with the change in Rouge versions.. (GitHub Pages is still on the older version..)
Is that worth addressing..?

[DirtyF]

Yeah it's clearly a visual regression there's an extra div with highlight class that wasn't here before because of Rouge 2.10.

[ashmaroli]

handled in #6389 ...