My ionic.css always overwrite when I run ionic lib update, why?

For example: I changed this line in my www/lib/ionic/css/ionic.css file:

background-color: #f39200;

But always that I run ionic lib update this line change to an older version:

background-color: #11c1f3;


you should NEVER edit files in lib/ or bower_components/ or node_modules/

use the app.ionic.css of scss if you need custom styling.