feat: support registering styles after component finalizes (#7069)

Author: tomivirkki

packages/vaadin-themable-mixin/test/post-finalize-styles-lit.test.ts

@@ -0,0 +1,19 @@
+import './post-finalize-styles.common.ts';
+import { css, LitElement } from 'lit';
+
+customElements.define(
+  'test-element',
+  class extends LitElement {
+    static get styles() {
+      return css`
+        :host {
+          display: block;
+        }
+      `;
+    }
+
+    render() {
+      return '';
+    }
+  },
+);

packages/vaadin-themable-mixin/test/post-finalize-styles-polymer.test.ts

@@ -0,0 +1,17 @@
+import './post-finalize-styles.common.ts';
+import { html, PolymerElement } from '@polymer/polymer/polymer-element.js';
+
+customElements.define(
+  'test-element',
+  class extends PolymerElement {
+    static get template() {
+      return html`
+        <style>
+          :host {
+            display: block;
+          }
+        </style>
+      `;
+    }
+  },
+);