Child theme - style.css http://www.wppicker.com/why-and-how-should-i-create-child-theme-for-wordpress/

child-theme-style.css

/*
Theme Name:     Twenty Fifteen Child
Theme URI:      http://example.com/
Description:    Child theme for the Twenty Fifteen theme
Author:         Your name here
Author URI:     http://example.com/about/
Template:       twentyfifteen
Version:        0.1.0
*/

@import url ("../twentyfifteen/style.css");