polygerrit-ui/app/elements/test/plugin.html

<dom-module id="my-plugin">
  <script>
    Gerrit.install(plugin =>
      plugin.registerStyleModule('app-theme', 'myplugin-app-theme')
    );
  </script>
</dom-module>

<dom-module id="myplugin-app-theme">
  <style>
    html {
      --primary-text-color: #F00BAA;
      --header-background-color: #F01BAA;
      --header-title-content: "MyGerrit";
      --footer-background-color: #F02BAA;
    }
  </style>
</dom-module>