This is a minor quibble but since the site's CSS is in review I thought I'd bring it up. I just found out, via this MSE answer, that if you ask a question and then comment on the answers below it, your username gets a slight background shading. I was surprised to hear this since in three years of using the site I'd never seen anything like it but if you look closely it is indeed there.
Look, for example, at an example from this post:
This shading is so slight it's essentially useless. Maybe trim the CSS to give it a bit more contrast? Otherwise just turn it off, there's not much point in having invisible features.
And while we're at this, I thought I'd open up the floor: are there other much-too-subtle changes in colouring which you think should get a fortifier on the CSS overhaul?