Increase contrast (and standardize color) of linked lines of code

Problem (from a SUS verbatim):

Linking to a block of code is a nightmare. As someone who is color blind, I can barely tell what lines are highlighted when I’m directly linked to a block of code [...] Gitlab UI doesn’t even scroll down to the lines highlighted.

Proposal

• Update to a higher contrast color (check across all themes)
• Standardize color across all files and diffs
• Make sure the page scrolls to the correct location when navigating directly to line of code