The only way to reference it is to use a CSS @import directive at the top of the visual.less file.
1. Copy the CSS file into your project, via copy and paste or npm/bower, etc. In this case, I copied the file to my style folder instead of referencing via npm because I had to make some changes to it to get it to work with Power BI.
2. Open up your style/visual.css file and add this line of code to the top. The path is relative to the root of the visual project.
Then just pbiviz package or pbiviz start to test it out.