Follow device colors (#4254)

Removing the color result to the browser choosing the good color depending of the system config.

p/themes/manifest.json

@@ -25,7 +25,5 @@
   ],
   "categories": ["news"],
   "display": "standalone",
-  "background_color": "#fff",
-  "theme_color": "#ffffff",
   "description": "FreshRSS is a self-hosted RSS feed aggregator."
 }