enhance: read theme color nodeinfo (#8977)

* provide theme color in nodeinfo metadata

* read theme color from nodeinfo

Prefer to read the theme color from the nodeinfo since it is more
performant than performing selector search on a DOM.

packages/backend/src/server/nodeinfo.ts

@@ -78,6 +78,7 @@ const nodeinfo2 = async () => {
 			enableEmail: meta.enableEmail,
 			enableServiceWorker: meta.enableServiceWorker,
 			proxyAccountName: proxyAccount ? proxyAccount.username : null,
+			themeColor: meta.themeColor || '#86b300',
 		},
 	};
 };