Add theming support.

- Added a number of common themes to our support matrix:
 - AtomOneDark
 - Dracula
 - VS
 - GitHub
 - GoogleCode
 - XCode
 - ... Admittedly these all were randomly picked, we could probably curate these better...
- Added a new `ThemeDialog` UI that can be accessed via `/theme`. It shows your currentlyt available themes and allows you to change them freely. It does **not**:
 - Save the theme between sessions
 - Allow you to hit escape
 - Show a preview prior to selection.
- These themes are from reacts highlight js library.

Fixes https://b.corp.google.com/issues/412797985

packages/cli/src/ui/themes/github.ts

@@ -0,0 +1,124 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { Theme } from './theme.js';
+
+export const GitHub: Theme = new Theme('GitHub', {
+  hljs: {
+    display: 'block',
+    overflowX: 'auto',
+    padding: '0.5em',
+    color: '#333',
+    background: '#f8f8f8',
+  },
+  'hljs-comment': {
+    color: '#998',
+    fontStyle: 'italic',
+  },
+  'hljs-quote': {
+    color: '#998',
+    fontStyle: 'italic',
+  },
+  'hljs-keyword': {
+    color: '#333',
+    fontWeight: 'bold',
+  },
+  'hljs-selector-tag': {
+    color: '#333',
+    fontWeight: 'bold',
+  },
+  'hljs-subst': {
+    color: '#333',
+    fontWeight: 'normal',
+  },
+  'hljs-number': {
+    color: '#008080',
+  },
+  'hljs-literal': {
+    color: '#008080',
+  },
+  'hljs-variable': {
+    color: '#008080',
+  },
+  'hljs-template-variable': {
+    color: '#008080',
+  },
+  'hljs-tag .hljs-attr': {
+    color: '#008080',
+  },
+  'hljs-string': {
+    color: '#d14',
+  },
+  'hljs-doctag': {
+    color: '#d14',
+  },
+  'hljs-title': {
+    color: '#900',
+    fontWeight: 'bold',
+  },
+  'hljs-section': {
+    color: '#900',
+    fontWeight: 'bold',
+  },
+  'hljs-selector-id': {
+    color: '#900',
+    fontWeight: 'bold',
+  },
+  'hljs-type': {
+    color: '#458',
+    fontWeight: 'bold',
+  },
+  'hljs-class .hljs-title': {
+    color: '#458',
+    fontWeight: 'bold',
+  },
+  'hljs-tag': {
+    color: '#000080',
+    fontWeight: 'normal',
+  },
+  'hljs-name': {
+    color: '#000080',
+    fontWeight: 'normal',
+  },
+  'hljs-attribute': {
+    color: '#000080',
+    fontWeight: 'normal',
+  },
+  'hljs-regexp': {
+    color: '#009926',
+  },
+  'hljs-link': {
+    color: '#009926',
+  },
+  'hljs-symbol': {
+    color: '#990073',
+  },
+  'hljs-bullet': {
+    color: '#990073',
+  },
+  'hljs-built_in': {
+    color: '#0086b3',
+  },
+  'hljs-builtin-name': {
+    color: '#0086b3',
+  },
+  'hljs-meta': {
+    color: '#999',
+    fontWeight: 'bold',
+  },
+  'hljs-deletion': {
+    background: '#fdd',
+  },
+  'hljs-addition': {
+    background: '#dfd',
+  },
+  'hljs-emphasis': {
+    fontStyle: 'italic',
+  },
+  'hljs-strong': {
+    fontWeight: 'bold',
+  },
+});