[UI] Increase diff byte highlight contrast.

Created by: cirosantilli

Currently I have to concentrate too much to find the byte highlight in a line, specially the additions. This would make it easier to spot.

Before:

After:

Would be even more important if we decide to do: http://feedback.gitlab.com/forums/176466-general/suggestions/6191324-highlight-bytes-words-in-diffs-on-adjacent-multi