functions.php

<?php

/**
 * Enqueue the styles from parent and child theme
 */
function twentyfifteen_flexbox_grid_parent_theme_enqueue_styles() {
	wp_enqueue_style( 'twentyfifteen-style', get_template_directory_uri() . '/style.css' );
	wp_enqueue_style( 'twentyfifteen-flexbox-grid-style',
		get_stylesheet_directory_uri() . '/style.css',
		array( 'twentyfifteen-style' )
	);

}
add_action( 'wp_enqueue_scripts', 'twentyfifteen_flexbox_grid_parent_theme_enqueue_styles' );

style.css

/*
Theme Name:     TwentyFifteen Flexbox Grid
Theme URI:      https://gist.github.com/2ndkauboy/360b29f20f43297fe7835718b3dc459e
Description:    Twentyfifteen child theme.
Author:         Bernhard Kau
Author URI:     https://kau-boys.de
Template:       twentyfifteen
Version:        0.1.0
*/

@media screen and (min-width: 38.75em) {
	.home .site-main {
		padding-left: 7.6923%;
		padding-right: 7.6923%;
	}

	.home .site-main {
		display: flex;
		flex-wrap: wrap;
		margin: auto -2%;
	}

	.home .site-main > article {
		flex: 1 0 200px;
		margin: 0 2% 5%;
		display: flex;
		flex-direction: column;
	}

	.home .site-main > article footer {
		margin-top: auto;
	}
}

@media screen and (min-width: 59.6875em) {
	.home .site-main {
		padding-left: 8.3333%;
		padding-right: 8.3333%;
	}
}