This is a demo tutorial. While there is no step-by-step commentary available (yet), the live demo below is fully functional. Feel free to open it for full source code.
This demo shows how to add an outline to line series using two methods: an additional (thicker) line series and a shadow.
Using additional series
See the Pen Line series with border by amCharts team (@amcharts) on CodePen.
Adding a line shadow
See the Pen Line series with border (using shadow) by amCharts team (@amcharts) on CodePen.