theme moved to plugin

packages/web/src/plugins/themes.js

@@ -0,0 +1,11 @@
+import * as light from './ThemeLight.svelte';
+import * as dark from './ThemeDark.svelte';
+
+export function buildThemes(plugins) {
+  const res = [light, dark];
+  for (const { content } of plugins) {
+    const { themes } = content;
+    if (themes) res.push(...themes);
+  }
+  return res;
+}