Make PWA menu bar the Flipper color

Summary:
{F1026310422}

I think it's quite nice having a matching title bar and buttons?

Changelog: The Flipper PWA now uses the Flipper purple as theme color

Reviewed By: lblasa

Differential Revision: D46646266

fbshipit-source-id: 1bc6573c3cd863678ceed5b9358a78d19606ee8a

desktop/static/manifest.template.json

@@ -6,7 +6,7 @@
     "start_url": "index.web.html",
     "display": "standalone",
     "background_color": "#fff",
-    "theme_color": "#4267B2",
+    "theme_color": "#722ed1",
     "orientation": "portrait-primary",
     "icons": [
       {