Check out example codes for "css stroke". It will help you in understanding the concepts better.

Code Example 1

<rect x="10" y="10" width="100" height="100" stroke="blue" fill="purple"
       fill-opacity="0.5" stroke-opacity="0.8"/>

Code Example 2

.module {
  stroke: black;
}

Learn ReactJs, React Native from akashmittal.com