Material Icons not showing up

I just published an application to the Android App Store, and after downloading from there, I realized that none of the Material Icons are showing up in the application. Any help is greatly appreciated.

This is what my ./client/head.html looks like

  <meta name="viewport" content="width=device-width, initial-scale=1.0, user-scalable=no" />
  <link href="" rel="stylesheet">
  <link rel="stylesheet" href="./main.css">