Track of range slider does not display in Firefox due to styled thumb button

Problem

We style the range input element's thumb (the round button). However, this causes the styling of the track on which it sits (the horizontal line) to become unstyled. This happens at least in the latest version of Firefox (66.0.4 at time of writing). Articles describe similar behaviour in Chrome although our styling seems to be broken in Chrome 74 entirely.

Solution

Add styling for the track that mirrors what browsers do (using the individual prefixes).

Alternatively we can remove the custom styling entirely because it's an element that isn't often used (the new gradient configuration is the only place I can come up with) and it looks fairly neutral in most default browser styling where it's known to work properly.