gerrit: update OpenDev theme CSS installation

registerStyleModule has been deprecated for a few releases, and is
removed in 3.8.  This inserts the style element directly, as suggested
by the Gerrit plugin documentation [1].

[1] https://gerrit-documentation.storage.googleapis.com/Documentation/3.7.2/pg-plugin-dev.html#low-level-style

Change-Id: I23600754dc35a9d455e611ac25a6162b2e973a03

playbooks/zuul/gerrit/files/plugins/opendev-theme-plugin.js

@@ -17,10 +17,9 @@
 
 'use strict';
 
-const openDevThemeStyle = document.createElement('dom-module');
+Gerrit.install(plugin => {
-openDevThemeStyle.innerHTML = `
+  const styleEl = document.createElement('style');
-      <template>
+  styleEl.innerHTML = `
-        <style>
     html {
       --header-title-content: "OpenDev";
       --header-icon: url("/static/opendev-sm.png");
@@ -29,11 +28,6 @@ openDevThemeStyle.innerHTML = `
       --header-border-bottom: 1px solid;
       --header-border-image: linear-gradient(to right, #ee265e,  #f9b8cb, #ee265e) 1;
     }
-        </style>
+  `;
-      </template>
+  document.head.appendChild(styleEl);
-`;
-openDevThemeStyle.register('opendev-site-theme');
-
-Gerrit.install(plugin => {
-    plugin.registerStyleModule('app-theme', 'opendev-site-theme');
 });