Tweak your contrast
Since day 1, users have been asking me change the contrast on diff highlighting — usually in opposite directions. It's pretty much impossible to pick a value that will satisfy everyone as the actual visual effect is too dependent on monitor quality (and settings) and the users' eyesight. Hence, along with turning animated transitions on or off, you can now also tweak Reviewable's contrast in the account settings dropdown:
At the moment this only affects diff highlighting, but as I rework the UI I will try to hook any other partial-contrast styles into the setting as well.