Make the slider in the Touch Bar window be gray, not blue #42

In v1, the slider was gray:

In v3, it's now blue:

<img width="1141" alt="Screen Shot 2019-05-13 at 00 15 55" src="https://user-images.githubusercontent.com/170270/57585579-4b9eba00-7514-11e9-9768-1d4dcff6de71.png">

I think that creates a small unnecessary distraction, so would be good to bring it back to be gray again.