".scrollbars-visible-always is undefined"

In an Atom package I have a LESS style sheet, which at one point includes the code

#outerContainer {
  width: 100%;
  height: 100%;
  position: relative;
  .scrollbars-visible-always(); // for Atom's scrollbar style
}

Access to scrollbars-visible-always is provided by @import "atom" at the top of the style sheet.

This works on my machine (Linux), and probably on some other people’s since the code is 5 month old and from ~1000 installations I would have expected to pop up sooner-

I have now received an error report from a Mac user

Error compiling Less stylesheet: /Users//.atom/packages/pdfjs-viewer/pdfjs/web/viewer.less

Line number: 604
.scrollbars-visible-always is undefined

but I have no idea what the reason for this could be. We are both on Atom 1.48.

Any ideas for the reason, whether it could actually be platform-related, or how to debug it?