Diff colors and font

Monospaced sans serif typeface:

• https://github.com/adobe-fonts/source-code-pro
• https://en.wikipedia.org/wiki/Source_Code_Pro

Colors to provide better contrast /saturation for users with colourblindness

Please unify UI colors/sizes across all diffs:

• no opacity used anymore, for colors please check gitlab1.atype->Styleguide Elements)
• extra whitespace after -/+ symbols for better visibility of code changes (accessibility issues)
• actions_symbols color and hover

/cc @jschatz1