Increase contrast ratio for markers via default styling

[danny-andrews]

🐛 Bug report

Current Behavior

The background color of the track and markers is #f2f2f2. This makes these elements difficult to distinguish on a white background. The contrast ratio is a measly 1.11:1!

Expected behavior

The track and markers should be styled to be more easily distinguishable on a white background. Playing around on https://webaim.org/resources/contrastchecker/, I came up with #D4D4D4, which gives a contrast ratio of ~1.5:1.

I realize the default styles are merely there to get a functional component, and are meant to be customized, but the default should at least be accessible.

[chaance]

I've since overhauled the slider's default styles quite a bit, but I'll double check to make sure this passes contrast checks. Thank you!