A few years ago, we switched our body text color from black to a dark gray. This resulted in a small but noticeable AdSense CTR improvement.
When we rolled out our new site design earlier this year, we forgot that important lesson. As a result, we were experiencing low CTR (and low eCPM) and we didn't know why.
Hellas noticed the difference and changed the body text color. Our CTR received that small but noticeable bump.
Small stuff like this can make big differences in the long term.